Trygve Helgaker

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• Penz, Markus; Herbst, Michael F.; Helgaker, Trygve & Laestadius, Andre (2026). Perspective on Moreau–Yosida regularization in density-functional theory. Electronic Structure. 8(2). doi: 10.1088/2516-1075/ae55eb. Full text in Research Archive
• Hillers-Bendtsen, Andreas Erbs; Johansen, Magnus Bukhave; Buchwald, Theo Juncker von; Mikkelsen, Kurt V.; Olsen, Jeppe & Jørgensen, Poul [Show all 7 contributors for this article] (2025). Cluster perturbation theory. XI. Excitation-energy series using a variational excitation-energy function. Journal of Chemical Physics. ISSN 0021-9606. 162(2). doi: 10.1063/5.0236908. Full text in Research Archive Show summary

  Traditionally, excitation energies in coupled-cluster (CC) theory have been calculated by solving the CC Jacobian eigenvalue equation. However, based on our recent work [Jørgensen et al., Sci. Adv. 10, eadn3454 (2024)], we propose a reformulation of the calculation of excitation energies where excitation energies are determined as a conventional molecular property. To this end, we introduce an excitation-energy function that depends on the CC Jacobian and the right and left eigenvectors for the Jacobian eigenvalue problem. This excitation-energy function is variational with respect to the right and left eigenvectors but not with respect to the cluster amplitudes. Instead, the cluster amplitudes satisfy the cluster-amplitude equations, and we set up an excitation-energy Lagrangian by adding to the excitation-energy function the cluster-amplitude equations with an undetermined multiplier for each cluster-amplitude constraint. The excitation-energy Lagrangian is variational in all its parameters. Based on the variational property of the Lagrangian, we have determined two quadratically convergent excitation-energy series: the total-order cluster-perturbation (tCP) and variational cluster-perturbation (vCP) excitation-energy series. Calculations of the excitation energies of three small molecules have shown that the vCP series is to be preferred over the tCP series. The test calculations have been carried out for CPS(D) expansions [targeting the CC singles-and-doubles (CCSD) wave function from the CC singles wave function] and the CPSD(T) expansion [targeting the CC singles-doubles-triples (CCSDT) wave function from the CCSD wave function]. For the S(D) and SD(T) orbital excitation space calculations, we obtain in the second vCP iteration excitation energies with a mean deviation from CCSD excitation energies of about 0.04 eV for the S(D) orbital spaces, and for the SD(T) orbital space calculation, we obtain a mean deviation from the CCSDT excitation energies of 0.001 eV.

• Jørgensen, Poul; Olsen, Jeppe; Johansen, Magnus Bukhave; Buchwald, Theo Juncker von; Hillers-Bendtsen, Andreas Erbs & Mikkelsen, Kurt V. [Show all 7 contributors for this article] (2024). A variational reformulation of molecular properties in electronic-structure theory. Science Advances. 10(17). doi: 10.1126/sciadv.adn3454. Full text in Research Archive Show summary

  Conventional quantum-mechanical calculations of molecular properties, such as dipole moments and electronic excitation energies, give errors that depend linearly on the error in the wave function. An exception is the electronic energy, whose error depends quadratically on the error in wave function. We here describe how all properties may be calculated with a quadratic error, by setting up a variational Lagrangian for the property of interest. Because the construction of the Lagrangian is less expensive than the calculation of the wave function, this approach substantially improves the accuracy of quantum-chemical calculations without increasing cost. As illustrated for excitation energies, this approach enables the accurate calculation of molecular properties for larger systems, with a short time-to-solution and in a manner well suited for modern computer architectures.

• Culpitt, Tanner Phillip; Tellgren, Erik; Peters, Laurens & Helgaker, Trygve (2024). Non-adiabatic coupling matrix elements in a magnetic field: Geometric gauge dependence and Berry phase. Journal of Chemical Physics. ISSN 0021-9606. 161(18). doi: 10.1063/5.0229854. Full text in Research Archive Show summary

  Non-adiabatic coupling matrix elements (NACMEs) are important in quantum chemistry, particularly for molecular dynamics methods such as surface hopping. However, NACMEs are gauge dependent. This presents a difficulty for their calculation in general, where there are no restrictions on the gauge function except that it be differentiable. These cases are relevant for complex-valued electronic wave functions, such as those that arise in the presence of a magnetic field or spin–orbit coupling. In addition, the Berry curvature and Berry force play an important role in molecular dynamics in a magnetic field and are also relevant in the context of spin–orbit coupling. For methods such as surface hopping, excited-state Berry curvatures will also be of interest. With this in mind, we have developed a scheme for the calculation of continuous, differentiable NACMEs as a function of the molecular geometry for complex-valued wave functions. We demonstrate the efficacy of the method using the H2 molecule at the full configuration–interaction (FCI) level of theory. In addition, ground- and excited-state Berry curvatures are computed for the first time using FCI theory. Finally, Berry phases are computed directly in terms of diagonal NACMEs.

• Teale, Andrew Michael; Huynh, Bang C.; Helgaker, Trygve & Tozer, David J. (2024). Classical Reaction Barriers in DFT: An Adiabatic-Connection Perspective. Journal of Chemical Theory and Computation. ISSN 1549-9618. p. 1–14. doi: 10.1021/acs.jctc.4c01038. Full text in Research Archive Show summary

  Classical reaction barriers in density-functional theory are considered from the perspective of the density-fixed adiabatic connection. A ‘reaction adiabatic-connection integrand’, ℛ𝜆, is introduced, where λ is the electron–electron interaction strength, for which ∫10ℛ𝜆d𝜆 equals the barrier, meaning the barrier can be easily visualized as the area under a plot of ℛ𝜆 vs λ. For five chemical reactions, plots of reference ℛ𝜆, calculated from Lieb maximizations at the coupled-cluster level of theory, are compared with approximate ℛ𝜆, calculated from common exchange–correlation functionals using coordinate scaling, for coupled-cluster densities. The comparison provides a simple way to visualize and understand functional-driven errors and trends in barriers from approximate functionals, while allowing a clean separation of the role of exchange and correlation contributions to the barrier. Specifically, the accuracy of ℛ0 is determined entirely by the accuracy of the exchange functional, while the shape of ℛ𝜆 is determined entirely by the correlation functional. The results clearly illustrate why the optimal amount of exact (orbital) exchange in hybrid functionals differs between reactions, including forward and reverse directions in the same reaction, and hence why simply introducing larger amounts of exact exchange may not be a reliable approach for improving barriers. Instead, the shape of ℛ𝜆 must be captured more accurately through more accurate correlation functionals, and the numerical data presented may be useful for this purpose. Density-driven errors are then considered, and possible cancellation with functional-driven errors in barriers─noted in prior studies when Hartree–Fock densities are used─is illustrated from the perspective of ℛ𝜆.

• Laestadius, Andre; Csirik, Mihaly Andras; Penz, Markus; Tancogne-Dejean, Nicolas; Ruggenthaler, Michael & Rubio, Angel [Show all 7 contributors for this article] (2024). Exchange-only virial relation from the adiabatic connection. Journal of Chemical Physics. ISSN 0021-9606. 160(8). doi: 10.1063/5.0184934. Full text in Research Archive Show summary

  The exchange-only virial relation due to Levy and Perdew is revisited. Invoking the adiabatic connection, we introduce the exchange energy in terms of the right-derivative of the universal density functional w.r.t. the coupling strength λ at λ = 0. This agrees with the Levy–Perdew definition of the exchange energy as a high-density limit of the full exchange–correlation energy. By relying on v-representability for a fixed density at varying coupling strength, we prove an exchange-only virial relation without an explicit local-exchange potential. Instead, the relation is in terms of a limit (λ ↘ 0) involving the exchange–correlation potential vλ xc, which exists by assumption of v-representability. On the other hand, a local-exchange potential vx is not warranted to exist as such a limit.

• Atsumi, Michiko; Zheng, Jia-Jia; Tellgren, Erik Ingemar; Sakaki, Shigeyoshi & Helgaker, Trygve (2023). Carbon dioxide adsorption to UiO-66: theoretical analysis of binding energy and NMR properties. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. doi: 10.1039/d3cp04033j. Full text in Research Archive
• Culpitt, Tanner Phillip; Peters, Laurens; Tellgren, Erik Ingemar & Helgaker, Trygve (2023). Time-dependent nuclear-electronic orbital Hartree-Fock theory in a strong uniform magnetic field. Journal of Chemical Physics. ISSN 0021-9606. 158(11). doi: 10.1063/5.0139675. Full text in Research Archive
• Peters, Laurens; Culpitt, Tanner Phillip; Tellgren, Erik Ingemar & Helgaker, Trygve (2023). Berry population analysis: Atomic charges from the Berry curvature in a magnetic field. Journal of Chemical Theory and Computation. ISSN 1549-9618. p. 1–12. doi: 10.1021/acs.jctc.2c01138. Full text in Research Archive Show summary

  The Berry curvature is essential in Born–Oppenheimer molecular dynamics, describing the screening of the nuclei by the electrons in a magnetic field. Parts of the Berry curvature can be understood as the external magnetic field multiplied by an effective charge so that the resulting Berry force behaves like a Lorentz force during the simulations. Here, we investigate whether these effective charges can provide insight into the electronic structure of a given molecule or, in other words, whether we can perform a population analysis based on the Berry curvature. To develop our approach, we first rewrite the Berry curvature in terms of charges that partially capture the effective charges and their dependencies on the nuclear velocities. With these Berry charges and charge fluctuations, we then construct our population analysis yielding atomic charges and overlap populations. Calculations at the Hartree–Fock level reveal that the atomic charges are similar to those obtained from atomic polar tensors. However, since we additionally obtain an estimate for the fluctuations of the charges and a partitioning of the atomic charges into contributions from all atoms, we conclude that the Berry population analysis is a useful alternative tool to analyze the electronic structures of molecules.

• Peters, Laurens; Tellgren, Erik Ingemar & Helgaker, Trygve (2023). Propagators for molecular dynamics in a magnetic field. Molecular Physics. ISSN 0026-8976. doi: 10.1080/00268976.2023.2259008. Full text in Research Archive Show summary

  Ab initio molecular dynamics in a magnetic field requires solving equations of motion with velocity-dependent forces – namely, the Lorentz force arising from the nuclear charges moving in a magnetic field and the Berry force arising from the shielding of these charges from the magnetic field by the surrounding electrons. In this work, we revisit two existing propagators for these equations of motion, the auxiliary-coordinates-and-momenta (ACM) propagator and the Tajima propagator (TAJ), and compare them with a new exponential (EXP) propagator based on the Magnus expansion. Additionally, we explore limits (for example, the zero-shielding limit), the implementation of higher-order integration schemes, and series truncation to reduce computational cost by carrying out simulations of a HeH+ model system for a wide range of field strengths. While being as efficient as the TAJ propagator, the EXP propagator is the only propagator that converges to both the schemes of Spreiter and Walter (derived for systems without shielding of the Lorentz force) and to the exact cyclotronic motion of a charged particle. Since it also performs best in our model simulations, we conclude that the EXP propagator is the recommended propagator for molecules in magnetic fields.

• Tellgren, Erik Ingemar; Culpitt, Tanner Phillip; Peters, Laurens & Helgaker, Trygve (2023). Molecular vibrations in the presence of velocity-dependent forces. Journal of Chemical Physics. ISSN 0021-9606. 158(12), p. 124124–124124. doi: 10.1063/5.0139684. Full text in Research Archive Show summary

  A semiclassical theory of small oscillations is developed for nuclei that are subject to velocity-dependent forces in addition to the usual interatomic forces. When the velocity-dependent forces are due to a strong magnetic field, novel effects arise—for example, the coupling of vibrational, rotational, and translational modes. The theory is first developed using Newtonian mechanics and we provide a simple quantification of the coupling between these types of modes. We also discuss the mathematical structure of the problem, which turns out to be a quadratic eigenvalue problem rather than a standard eigenvalue problem. The theory is then re-derived using the Hamiltonian formalism, which brings additional insight, including a close analogy to the quantum-mechanical treatment of the problem. Finally, we provide numerical examples for the H2, HT, and HCN molecules in a strong magnetic field.

• Helgaker, Trygve & Teale, Andrew M. (2022). Lieb variation principle in density-functional theory. In Frank, Rupert L.; Laptev, Ari; Lewin, Mathieu & Seiringer, Robert (Ed.), The Physics and Mathematics of Elliot Lieb. European Mathematical Society (EMS) Press. ISSN 9783985470211. p. 527–559. doi: 10.4171/90-1/22. Full text in Research Archive
• Dembiński, Stanisław; Karwowski, Jacek; Szudy, Józef & Helgaker, Trygve (2022). Lutosław Wolniewicz (1930–2020). Molecular Physics. ISSN 0026-8976. doi: 10.1080/00268976.2021.2024904. Full text in Research Archive
• Pemberton, Miles J.; Irons, Tom J. P.; Helgaker, Trygve & Teale, Andrew Michael (2022). Revealing the exotic structure of molecules in strong magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 156(20). doi: 10.1063/5.0092520. Full text in Research Archive Show summary

  A novel implementation for the calculation of molecular gradients under strong magnetic fields is employed at the current-density functional theory level to optimize the geometries of molecular structures, which change significantly under these conditions. An analog of the ab initio random structure search is utilized to determine the ground-state equilibrium geometries for He n and CH n systems at high magnetic field strengths, revealing the most stable structures to be those in high-spin states with a planar geometry aligned perpendicular to the field. The electron and current densities for these systems have also been investigated to develop an explanation of chemical bonding in the strong field regime, providing an insight into the exotic chemistry present in these extreme environments.

• Monzel, Laurenz; Pausch, Ansgar; Peters, Laurens; Tellgren, Erik Ingemar; Helgaker, Trygve & Klopper, Wim (2022). Molecular dynamics of linear molecules in strong magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 157(5). doi: 10.1063/5.0097800. Full text in Research Archive Show summary

  Molecular rotations and vibrations have been extensively studied by chemists for decades, both experimentally using spectroscopic methods and theoretically with the help of quantum chemistry. However, the theoretical investigation of molecular rotations and vibrations in strong magnetic fields requires computationally more demanding tools. As such, proper calculations of rotational and vibrational spectra were not feasible up until very recently. In this work, we present rotational and vibrational spectra for two small linear molecules, H 2 and LiH, in strong magnetic fields. By treating the nuclei as classical particles, trajectories for rotations and vibrations are simulated from ab initio molecular dynamics. Born–Oppenheimer potential energy surfaces are calculated at the Hartree–Fock and MP2 levels of theory using London atomic orbitals to ensure gauge origin invariance. For the calculation of nuclear trajectories, a highly efficient Tajima propagator is introduced, incorporating the Berry curvature tensor accounting for the screening of nuclear charges.

• Culpitt, Tanner Phillip; Peters, Laurens; Tellgren, Erik Ingemar & Helgaker, Trygve (2022). Analytic calculation of the Berry curvature and diagonal Born-Oppenheimer correction for molecular systems in uniform magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 156(4). doi: 10.1063/5.0079304. Full text in Research Archive Show summary

  The diagonal nonadiabatic term arising from the Born–Oppenheimer wave function ansatz contains contributions from a vector and scalar potential. The former is provably zero when the wave function can be taken to be real valued, and the latter, known as the diagonal Born–Oppenheimer correction (DBOC), is typically small in magnitude. Therefore, unless high accuracy is sought, the diagonal nonadiabatic term is usually neglected when calculating molecular properties. In the presence of a magnetic field, the wave function is generally complex, and the geometric vector potential gives rise to a screening force that is qualitatively important for molecular dynamics. This screening force is written in terms of the Berry curvature and is added to the bare Lorentz force acting on the nuclei in the presence of the field. In this work, we derive analytic expressions for the Berry curvature and DBOC using both first- and second-quantization formalisms for the case of generalized and restricted Hartree–Fock theories in a uniform magnetic field. The Berry curvature and DBOC are calculated as a function of the magnetic field strength and the bond distance for the ground-state singlets of H2, LiH, BH, and CH+. We also examine the stability and time-reversal symmetry of the underlying self-consistent field solutions. The character of the DBOC and Berry curvature is found to depend on the magnetic field and varies between molecules. We also identify instances of broken time-reversal symmetry for the dissociation curves of BH and CH+.

• Skjelstad, Bastian; Helgaker, Trygve; Maeda, Satoshi & Badia, David Balcells (2022). Oxyl Character and Methane Hydroxylation Mechanism in Heterometallic M(O)Co3O4Cubanes (M = Cr, Mn, Fe, Mo, Tc, Ru, and Rh). ACS Catalysis. 12(19), p. 12326–12335. doi: 10.1021/acscatal.2c03748. Full text in Research Archive Show summary

  C–H activation in alkanes poses a major challenge in chemistry due to the inert character of this bond, manifesting the necessity of improved catalysts. Although various metal–oxo complexes are known to facilitate alkane hydroxylation, probing the mechanistic nature of the reaction is difficult due to the extremely fast rebound of the radical intermediate in the postulated oxygen-rebound pathway. Automated reaction mechanism discovery methods, such as the artificial force induced reaction (AFIR) method, enable the efficient exploration of both expected and unexpected reaction pathways, revealing the reaction mechanism. Here, we employed this approach combined with density-functional theory (DFT) to investigate the structure and reactivity of heterometallic cubane complexes similar to the oxygen-evolving complex of photosystem II. For a series of M(O)Co3O4 cubanes, where M(O) is a terminal oxo with M = Cr, Mn, Fe, Mo, Tc, Ru, and Rh, we computed the stability of the possible spin states and the radical (i.e., oxyl) character of the M(O) moiety as a measure of their potential activity in the catalytic hydroxylation of alkanes. DFT calculations on these reactions promoted by Ru(O)Co3O4 and Fe(O)Co3O4 suggest that the latter promotes the hydroxylation of methane with a rate-determining H-abstraction barrier of 24.6 kcal/mol. The moderate height of this barrier, together with the low cost and low toxicity of iron and cobalt, suggests that the Fe(O)Co3O4 cubane is a promising candidate for the catalytic oxidation of methane to methanol. AFIR calculations showed that the oxygen-rebound pathway yields the lowest-energy profile, thus validating this mechanism for the hydroxylation of alkanes by heterometallic cubanes. Furthermore, unexpected intermediates in which the methyl radical couples with either the metal center or the bridging oxo ligands were also observed.

• Teale, Andrew Michael; Helgaker, Trygve; Savin, Andreas; Adamo, Carlo; Aradi, Balint & Arbuznikov, Alexei V. [Show all 70 contributors for this article] (2022). DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. doi: 10.1039/d2cp02827a. Full text in Research Archive Show summary

  In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022.

• Peters, Laurens; Culpitt, Tanner Phillip; Tellgren, Erik Ingemar & Helgaker, Trygve (2022). Magnetic-translational sum rule and approximate models of the molecular Berry curvature. Journal of Chemical Physics. ISSN 0021-9606. 157(13). doi: 10.1063/5.0112943. Full text in Research Archive Show summary

  The Berry connection and curvature are key components of electronic structure calculations for atoms and molecules in magnetic fields. They ensure the correct translational behavior of the effective nuclear Hamiltonian and the correct center-of-mass motion during molecular dynamics in these environments. In this work, we demonstrate how these properties of the Berry connection and curvature arise from the translational symmetry of the electronic wave function and how they are fully captured by a finite basis set of London orbitals but not by standard Gaussian basis sets. This is illustrated by a series of Hartree–Fock calculations on small molecules in different basis sets. Based on the resulting physical interpretation of the Berry curvature as the shielding of the nuclei by the electrons, we introduce and test a series of approximations using the Mulliken fragmentation scheme of the electron density. These approximations will be particularly useful in ab initio molecular dynamics calculations in a magnetic field since they reduce the computational cost, while recovering the correct physics and up to 95% of the exact Berry curvature.

• Ryley, Matthew S.; Withnall, Michael; Irons, Tom J. P.; Helgaker, Trygve & Teale, Andrew Michael (2021). Robust All-Electron Optimization in Orbital-Free Density-Functional Theory Using the Trust-Region Image Method. Journal of Physical Chemistry A. ISSN 1089-5639. 125(1), p. 459–475. doi: 10.1021/acs.jpca.0c09502. Full text in Research Archive
• Kvaal, Simen; Laestadius, Andre; Tellgren, Erik Ingemar & Helgaker, Trygve (2021). Lower Semicontinuity of the Universal Functional in Paramagnetic Current-Density Functional Theory. The Journal of Physical Chemistry Letters. 12(5), p. 1421–1425. doi: 10.1021/acs.jpclett.0c03422. Full text in Research Archive Show summary

  A cornerstone of current–density functional theory (CDFT) in its paramagnetic formulation is proven. After a brief outline of the mathematical structure of CDFT, the lower semicontinuity and expectation-valuedness of the CDFT constrained-search functional is proven, meaning that there is always a minimizing density matrix in the CDFT constrained-search universal density functional. These results place the mathematical framework of CDFT on the same footing as that of standard DFT.

• Peters, Laurens; Culpitt, Tanner Phillip; Monzel, Laurenz; Tellgren, Erik Ingemar & Helgaker, Trygve (2021). Ab Initio molecular dynamics with screened Lorentz forces. II. Efficient propagators and rovibrational spectra in strong magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 155. doi: 10.1063/5.0056235. Full text in Research Archive
• Culpitt, Tanner Phillip; Peters, Laurens; Tellgren, Erik Ingemar & Helgaker, Trygve (2021). Ab initio molecular dynamics with screened Lorentz forces. I. Calculation and atomic charge interpretation of Berry curvature. Journal of Chemical Physics. ISSN 0021-9606. 155(2). doi: 10.1063/5.0055388. Full text in Research Archive Show summary

  The dynamics of a molecule in a magnetic field is significantly different from its zero-field counterpart. One important difference in the presence of a field is the Lorentz force acting on the nuclei, which can be decomposed as the sum of the bare nuclear Lorentz force and a screening force due to the electrons. This screening force is calculated from the Berry curvature and can change the dynamics qualitatively. It is therefore important to include the contributions from the Berry curvature in molecular dynamics simulations in a magnetic field. In this work, we present a scheme for calculating the Berry curvature numerically using a finite-difference technique, addressing challenges related to the arbitrary global phase of the wave function. The Berry curvature is calculated as a function of bond distance for H2 at the restricted and unrestricted Hartree–Fock levels of theory and for CH+ as a function of the magnetic field strength at the restricted Hartree–Fock level of theory. The calculations are carried out using basis sets of contracted Gaussian functions equipped with London phase factors (London orbitals) to ensure gauge-origin invariance. In this paper, we also interpret the Berry curvature in terms of atomic charges and discuss its convergence in basis sets with and without London phase factors. The calculation of the Berry curvature allows for its inclusion in ab initio molecular dynamics simulations in a magnetic field.

• Pasteka, Lukáš Félix; Helgaker, Trygve; Saue, Trond; Sundholm, Dage; Werner, Hans-Joachim & Hasanbulli, Mustafa [Show all 8 contributors for this article] (2020). Atoms and molecules in soft confinement potentials. Molecular Physics. ISSN 0026-8976. p. 1–23. doi: 10.1080/00268976.2020.1730989. Full text in Research Archive
• Castro, Abril C; Balcells, David; Repisky, Michal; Helgaker, Trygve & Cascella, Michele (2020). First-principles calculation of 1H NMR chemical shifts of complex metal polyhydrides: The essential inclusion of relativity and dynamics. Inorganic Chemistry. ISSN 0020-1669. 59(23), p. 17509–17518. doi: 10.1021/acs.inorgchem.0c02753. Full text in Research Archive Show summary

  ¹H NMR spectroscopy has become an important technique for the characterization of transition-metal hydride complexes, whose metal-bound hydrides are often difficult to locate by X-ray diffraction. In this regard, the accurate prediction of ¹H NMR chemical shifts provides a useful, but challenging, strategy to help in the interpretation of the experimental spectra. In this work, we establish a density-functional-theory protocol that includes relativistic, solvent, and dynamic effects at a high level of theory, allowing us to report an accurate and reliable interpretation of ¹H NMR hydride chemical shifts of iridium polyhydride complexes. In particular, we have studied in detail the hydride chemical shifts of the [Ir₆(IMe)₈(CO)₂H₁₄]²⁺ complex in order to validate previous assignments. The computed ¹H NMR chemical shifts are strongly dependent on the relativistic treatment, the choice of the DFT exchange–correlation functional, and the conformational dynamics. By combining a fully relativistic four-component electronic-structure treatment with ab initio molecular dynamics, we were able to reliably model both the terminal and bridging hydride chemical shifts and to show that two NMR hydride signals were inversely assigned in the experiment.

• Irons, Tom J P; Spence, Lucy; Speake, Gregoire David; Speake, Benjamin T.; Helgaker, Trygve & Teale, Andrew Michael (2020). Analyzing Magnetically Induced Currents in Molecular Systems Using Current-Density-Functional Theory. Journal of Physical Chemistry A. ISSN 1089-5639. 124, p. 1321–1333. doi: 10.1021/acs.jpca.9b10833. Full text in Research Archive
• Brandenburg, Jan Gerit; Burke, Kieron; Fromager, Emmanuel; Gatti, Matteo; Giarrusso, Sara & Gidopoulos, Nikitas I. [Show all 29 contributors for this article] (2020). New approaches to study excited states in density functional theory: general discussion. Faraday discussions. ISSN 1359-6640. 224, p. 483–508. doi: 10.1039/D0FD90026E. Full text in Research Archive
• Fromager, Emmanuel; Gidopoulos, Nikitas I.; Gori-Giorgi, Paola; Helgaker, Trygve; Loos, Pierre-François & Malcomson, Thomas [Show all 11 contributors for this article] (2020). Strong correlation in density functional theory: general discussion. Faraday discussions. ISSN 1359-6640. 224, p. 373–381. doi: 10.1039/D0FD90025G. Full text in Research Archive
• Brandenburg, Jan Gerit; Burke, Kieron; Civalleri, Bartolomeo; Cole, Daniel J.; Csanyi, Gabor & Gregoire, David [Show all 27 contributors for this article] (2020). Challenges for large scale simulation: general discussion. Faraday discussions. ISSN 1359-6640. 224, p. 309–332. doi: 10.1039/D0FD90024A. Full text in Research Archive
• Brandenburg, Jan Gerit; Burke, Kieron; Cancio, Antonio; Erhard, Jannis; Fromager, Emmanuel & Ghosal, Abhisek [Show all 25 contributors for this article] (2020). New density-functional approximations and beyond: general discussion. Faraday discussions. ISSN 1359-6640. 224, p. 166–200. doi: 10.1039/D0FD90023K. Full text in Research Archive
• Austad, Jon; Borgoo, Alex; Tellgren, Erik & Helgaker, Trygve (2020). Bonding in the helium dimer in strong magnetic fields: The role of spin and angular momentum. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 22(41), p. 23502–23521. doi: 10.1039/d0cp03259j. Full text in Research Archive
• Olsen, Jógvan Magnus Haugaard; Reine, Simen Sommerfelt; Vahtras, Olav; Kjellgren, Erik; Reinholdt, Peter & Dundas, Karen O.H [Show all 25 contributors for this article] (2020). Dalton Project: A Python platform for molecular- and electronic-structure simulations of complex systems. Journal of Chemical Physics. ISSN 0021-9606. 152(21). doi: 10.1063/1.5144298. Full text in Research Archive Show summary

  The <i>Dalton Project</i> provides a uniform platform access to the underlying full-fledged quantum chemistry codes Dalton and LSDalton as well as the PyFraME package for automatized fragmentation and parameterization of complex molecular environments. The platform is written in Python and defines a means for library communication and interaction. Intermediate data such as integrals are exposed to the platform and made accessible to the user in the form of NumPy arrays, and the resulting data are extracted, analyzed, and visualized. Complex computational protocols that may, for instance, arise due to a need for environment fragmentation and configuration-space sampling of biochemical systems are readily assisted by the platform. The platform is designed to host additional software libraries and will serve as a hub for future modular software development efforts in the distributed Dalton community.

• Adamowicz, Ludwik; Stanke, Monika; Tellgren, Erik & Helgaker, Trygve (2020). A quantum-mechanical non-Born–Oppenheimer model of a molecule in a strong magnetic field. Chemical Physics Letters. ISSN 0009-2614. 761, p. 1–7. doi: 10.1016/j.cplett.2020.138041. Full text in Research Archive Show summary

  Interactions of a stationary external magnetic field with the spin and orbital magnetic momenta of a molecule are included in the quantum mechanical model where the Born–Oppenheimer approximation is not assumed. The model is used to calculate some of the lowest-lying internal bound states of the molecule for various strengths of the magnetic field. All-particle explicitly correlated Gaussian functions are used in the calculations.

• Holzer, Christof; Teale, Andrew Michael; Hampe, Florian; Stopkowicz, Stella; Helgaker, Trygve & Klopper, Wim (2019). GW quasiparticle energies of atoms in strong magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 150(21), p. 214112-1–214112-9. doi: 10.1063/1.5093396. Full text in Research Archive
• Laestadius, Andre; Tellgren, Erik; Penz, Markus; Ruggenthaler, Michael; Kvaal, Simen & Helgaker, Trygve (2019). Kohn–Sham Theory with Paramagnetic Currents: Compatibility and Functional Differentiability. Journal of Chemical Theory and Computation. ISSN 1549-9618. 15(7), p. 4003–4020. doi: 10.1021/acs.jctc.9b00141. Full text in Research Archive
• Castro, Abril C.; Fliegl, Heike; Cascella, Michele; Helgaker, Trygve; Repisky, Michal & Komorovsky, Stanislav [Show all 9 contributors for this article] (2019). Four-component relativistic 31P NMR calculations for: Trans -platinum(ii) complexes: Importance of the solvent and dynamics in spectral simulations. Dalton Transactions. ISSN 1477-9226. 48(23), p. 8076–8083. doi: 10.1039/c9dt00570f. Full text in Research Archive Show summary

  We report a combined experimental-theoretical study on the 31P NMR chemical shift for a number of trans-platinum(II) complexes. Validity and reliability of the 31P NMR chemical shift calculations are examined by comparing with the experimental data. A successful computational protocol for the accurate prediction of the 31P NMR chemical shifts was established for trans-[PtCl2(dma)PPh3] (dma = dimethylamine) complex. The reliability of the computed values is shown to be critically dependent on the level of relativistic effects (two-component vs. four component), choice of density functional, dynamical averaging, and solvation effects. Snapshots from ab initio molecular dynamics simulations were used to identify those solvent molecules which show the largest interactions with the platinum complex, through inspection by the non-covalent interaction program. We observe satisfactory accuracy from the full four-component matrix Dirac-Kohn-Sham method (mDKS) based on the Dirac-Coulomb Hamiltonian, in conjunction with the KT2 density functional, and dynamical averaging with explicit solvent molecules

• Reimann, Sarah; Borgoo, Alex; Austad, Jon; Tellgren, Erik; Teale, Andrew Michael & Helgaker, Trygve [Show all 7 contributors for this article] (2019). Kohn–Sham energy decomposition for molecules in a magnetic field. Molecular Physics. ISSN 0026-8976. 117(1), p. 97–109. doi: 10.1080/00268976.2018.1495849. Full text in Research Archive
• Holzer, Christof; Gui, Xin; Harding, Michael E.; Kresse, Georg; Helgaker, Trygve & Klopper, Wim (2018). Bethe-Salpeter correlation energies of atoms and molecules. Journal of Chemical Physics. ISSN 0021-9606. 149(14), p. 144106-1–144106-9. doi: 10.1063/1.5047030. Full text in Research Archive
• Tellgren, Erik; Laestadius, Andre; Helgaker, Trygve; Kvaal, Simen & Teale, Andrew Michael (2018). Uniform magnetic fields in density-functional theory. Journal of Chemical Physics. ISSN 0021-9606. 148(2), p. 1–19. doi: 10.1063/1.5007300. Full text in Research Archive
• Adamowicz, Ludwik; Stanke, Monika; Tellgren, Erik & Helgaker, Trygve (2018). A computational quantum-mechanical model of a molecular magnetic trap. Journal of Chemical Physics. ISSN 0021-9606. 149(24), p. 244112-1–244112-11. doi: 10.1063/1.5055767. Full text in Research Archive
• Laestadius, Andre; Penz, Markus; Tellgren, Erik; Ruggenthaler, Michael; Kvaal, Simen & Helgaker, Trygve (2018). Generalized Kohn-Sham iteration on Banach spaces. Journal of Chemical Physics. ISSN 0021-9606. 149(16), p. 1–9. doi: 10.1063/1.5037790. Full text in Research Archive Show summary

  A detailed account of the Kohn–Sham (KS) algorithm from quantum chemistry, formulated rigorously in the very general setting of convex analysis on Banach spaces, is given here. Starting from a Levy–Lieb-type functional, its convex and lower semi-continuous extension is regularized to obtain differentiability. This extra layer allows us to rigorously introduce, in contrast to the common unregularized approach, a well-defined KS iteration scheme. Convergence in a weak sense is then proven. This generalized formulation is applicable to a wide range of different density-functional theories and possibly even to models outside of quantum mechanics.

• Rebolini, Elisa; Teale, Andrew Michael; Helgaker, Trygve; Savin, Andreas & Toulouse, Julien (2018). Excitation energies from Gorling?Levy perturbation theory along the range-separated adiabatic connection. Molecular Physics. ISSN 0026-8976. 116(11), p. 1443–1451. doi: 10.1080/00268976.2017.1422811. Full text in Research Archive
• Adamowicz, Ludwik; Stanke, Monika; Tellgren, Erik & Helgaker, Trygve (2017). Explicitly-correlated non-born-oppenheimer calculations of the HD molecule in a strong magnetic field. Chemical Physics Letters. ISSN 0009-2614. 682, p. 87–90. doi: 10.1016/j.cplett.2017.06.016. Full text in Research Archive
• Reimann, Sarah; Borgoo, Alex; Tellgren, Erik; Teale, Andrew Michael & Helgaker, Trygve (2017). Magnetic-Field Density-Functional Theory (BDFT): Lessons from the Adiabatic Connection. Journal of Chemical Theory and Computation. ISSN 1549-9618. 13(9), p. 4089–4100. doi: 10.1021/acs.jctc.7b00295. Full text in Research Archive Show summary

  We study the effects of magnetic fields in the context of magnetic field density-functional theory (BDFT), where the energy is a functional of the electron density ρ and the magnetic field B. We show that this approach is a worthwhile alternative to current-density functional theory (CDFT) and may provide a viable route to the study of many magnetic phenomena using density-functional theory (DFT). The relationship between BDFT and CDFT is developed and clarified within the framework of the four-way correspondence of saddle functions and their convex and concave parents in convex analysis. By decomposing the energy into its Kohn–Sham components, we demonstrate that the magnetizability is mainly determined by those energy components that are related to the density. For existing density functional approximations, this implies that, for the magnetizability, improvements of the density will be more beneficial than introducing a magnetic-field dependence in the correlation functional. However, once a good charge density is achieved, we show that high accuracy is likely only obtainable by including magnetic-field dependence. We demonstrate that adiabatic-connection (AC) curves at different field strengths resemble one another closely provided each curve is calculated at the equilibrium geometry of that field strength. In contrast, if all AC curves are calculated at the equilibrium geometry of the field-free system, then the curves change strongly with increasing field strength due to the increasing importance of static correlation. This holds also for density functional approximations, for which we demonstrate that the main error encountered in the presence of a field is already present at zero field strength, indicating that density-functional approximations may be applied to systems in strong fields, without the need to treat additional static correlation. The final version of this research has been published in Journal of Chemical Theory and Computation. © 2017 American Chemical Society

• Irons, Tom J P; Furness, James W.; Ryley, Matthew S.; Zemen, Jan; Helgaker, Trygve & Teale, Andrew Michael (2017). Connections between variation principles at the interface of wave-function and density-functional theories. Journal of Chemical Physics. ISSN 0021-9606. 147(13). doi: 10.1063/1.4985883. Full text in Research Archive Show summary

  A recently proposed variation principle [N. I. Gidopoulos, Phys. Rev. A 83, 040502(R) (2011)] for the determination of Kohn–Sham effective potentials is examined and extended to arbitrary electron-interaction strengths and to mixed states. Comparisons are drawn with Lieb’s convex-conjugate functional, which allows for the determination of a potential associated with a given electron density by maximization, yielding the Kohn–Sham potential for a non-interacting system. The mathematical structure of the two functionals is shown to be intrinsically related; the variation principle put forward by Gidopoulos may be expressed in terms of the Lieb functional. The equivalence between the information obtained from the two approaches is illustrated numerically by their implementation in a common framework. This article was originally published in the Journal of Chemical Physics. © 2017 AIP Publishing

• Teale, Andrew Michael; Helgaker, Trygve & Savin, Andreas (2016). Alternative Representations of the Correlation Energy in Density-Functional Theory: A Kinetic-Energy Based Adiabatic Connection. Journal of the Chinese Chemical Society (Taipei). ISSN 0009-4536. 63(1), p. 121–128. doi: 10.1002/jccs.201500132. Full text in Research Archive
• Kumar, Chandan; Kjærgaard, Thomas; Helgaker, Trygve & Fliegl, Heike (2016). Nuclei-selected atomic orbital response theory formulation of NMR shielding tensors using density-fitting. Journal of Chemical Physics. ISSN 0021-9606. 145(23), p. 1–10. doi: 10.1063/1.4972212. Full text in Research Archive
• Rebolini, Elisa; Izsak, Robert; Reine, Simen Sommerfelt; Helgaker, Trygve & Pedersen, Thomas Bondo (2016). Comparison of Three Efficient Approximate Exact-Exchange Algorithms: The Chain-of-Spheres Algorithm, Pair-Atomic Resolution-of-the-Identity Method, and Auxiliary Density Matrix Method. Journal of Chemical Theory and Computation. ISSN 1549-9618. 12, p. 3514–3522. doi: 10.1021/acs.jctc.6b00074. Full text in Research Archive
• Furness, James W.; Ekström, Ulf Egil; Helgaker, Trygve & Teale, Andrew Michael (2016). Electron localisation function in current-density-functional theory. Molecular Physics. ISSN 0026-8976. 114(7-8), p. 1415–1422. doi: 10.1080/00268976.2015.1133859. Full text in Research Archive
• Helgaker, Trygve; Jaszunski, Michal & Swider, Pawel (2016). Calculation of NMR Spin-Spin Coupling Constants in Strychnine. Journal of Organic Chemistry. ISSN 0022-3263. 81(22), p. 11496–11500. doi: 10.1021/acs.joc.6b02157. Full text in Research Archive
• Kvaal, Simen & Helgaker, Trygve (2015). Ground-state densities from the Rayleigh-Ritz variation principle and from density-functional theory. Journal of Chemical Physics. ISSN 0021-9606. 143(18). doi: 10.1063/1.4934797. Full text in Research Archive
• Reimann, Sarah; Ekström, Ulf Egil; Stopkowicz, Stella; Teale, Andrew Michael; Borgoo, Alex & Helgaker, Trygve (2015). The importance of current contributions to shielding constants in density-functional theory. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 17(28), p. 18834–18842. doi: 10.1039/c5cp02682b. Full text in Research Archive
• Rebolini, Elisa; Toulouse, Julien; Teale, Andrew Michael; Helgaker, Trygve & Savin, Andreas (2015). Excited states from a range-separated density-functional perturbation theory. Molecular Physics. ISSN 0026-8976. 113(13-14), p. 1740–1749. doi: 10.1080/00268976.2015.1011248. Full text in Research Archive
• Borgoo, Alex; Teale, Andrew Michael & Helgaker, Trygve (2015). Excitation energies from ensemble DFT. AIP Conference Proceedings. ISSN 0094-243X. 1702:090049. doi: 10.1063/1.4938857. Full text in Research Archive
• Cheng, Chi Y.; Ryley, Matthew S.; Peach, Michael J.G.; Tozer, David J.; Helgaker, Trygve & Teale, Andrew Michael (2015). Molecular properties in the Tamm–Dancoff approximation: indirect nuclear spin–spin coupling constants. Molecular Physics. ISSN 0026-8976. 113(13-14), p. 1937–1951. doi: 10.1080/00268976.2015.1024182. Full text in Research Archive
• Hoffmann, Mark R. & Helgaker, Trygve (2015). Use of Density Functional Theory Orbitals in the GVVPT2 Variant of Second-Order Multistate Multireference Perturbation Theory. Journal of Physical Chemistry A. ISSN 1089-5639. 119, p. 1548–1553. doi: 10.1021/jp507554v. Full text in Research Archive
• Peach, Michael J. G.; Teale, Andrew Michael; Helgaker, Trygve & Tozer, David J. (2015). Fractional electron loss in approximate DFT and Hartree-Fock theory. Journal of Chemical Theory and Computation. ISSN 1549-9618. 11(11), p. 5262–5268. doi: 10.1021/acs.jctc.5b00804. Full text in Research Archive
• Rebolini, Elisa; Toulouse, Julien; Teale, Andrew Michael; Helgaker, Trygve & Savin, A (2015). Calculating excitation energies by extrapolation along adiabatic connections. Physical Review A. Atomic, Molecular, and Optical Physics (PRA). ISSN 1050-2947. 91. doi: 10.1103/PhysRevA.91.032519. Full text in Research Archive Show summary

  In this paper, an alternative method to range-separated linear-response time-dependent density-functional theory and perturbation theory is proposed to improve the estimation of the energies of a physical system from the energies of a partially interacting system. Starting from the analysis of the Taylor expansion of the energies of the partially interacting system around the physical system, we use an extrapolation scheme to improve the estimation of the energies of the physical system at an intermediate point of the range-separated or linear adiabatic connection where either the electron–electron interaction is scaled or only the long-range part of the Coulomb interaction is included. The extrapolation scheme is first applied to the range-separated energies of the helium and beryllium atoms and of the hydrogen molecule at its equilibrium and stretched geometries. It improves significantly the convergence rate of the energies toward their exact limit with respect to the range-separation parameter. The range-separated extrapolation scheme is compared with a similar approach for the linear adiabatic connection, highlighting the relative strengths and weaknesses of each approach. © 2015 American Physical Society

• Ryding, Mauritz Johan Olof; Izsak, Robert; Merlot, Patrick; Reine, Simen Sommerfelt; Helgaker, Trygve & Uggerud, Einar (2015). Geometry of the magic number H+(H2O)21 water cluster by proxy. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 17, p. 5466–5473. doi: 10.1039/C4CP05246C. Full text in Research Archive
• Stopkowicz, Stella; Gauss, Jürgen; Lange, Kai Kaarvann; Tellgren, Erik & Helgaker, Trygve (2015). Coupled-cluster theory for atoms and molecules in strong magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 143:074110(7). doi: 10.1063/1.4928056. Full text in Research Archive
• Furness, James W.; Verbeke, Joachim; Tellgren, Erik; Stopkowicz, Stella; Ekström, Ulf Egil & Helgaker, Trygve [Show all 7 contributors for this article] (2015). Current density functional theory using meta-generalized gradient exchange-correlation functionals. Journal of Chemical Theory and Computation. ISSN 1549-9618. 11(9), p. 4169–4181. doi: 10.1021/acs.jctc.5b00535. Full text in Research Archive
• Adamowicz, Ludwik; Tellgren, Erik & Helgaker, Trygve (2015). Non-Born-Oppenheimer calculations of the HD molecule in a strong magnetic field. Chemical Physics Letters. ISSN 0009-2614. 639, p. 295–299. doi: 10.1016/j.cplett.2015.09.051. Full text in Research Archive
• Kvaal, Simen; Ekström, Ulf Egil; Teale, Andrew Michael & Helgaker, Trygve (2014). Differentiable but exact formulation of density-functional theory. Journal of Chemical Physics. ISSN 0021-9606. 140(18). doi: 10.1063/1.4867005. Full text in Research Archive
• Tellgren, Erik; Teale, Andrew Michael; Furness, J.W.; Lange, Kai Kaarvann; Ekström, Ulf Egil & Helgaker, Trygve (2014). Non-perturbative calculation of molecular magnetic properties within current-density functional theory. Journal of Chemical Physics. ISSN 0021-9606. 140(3). doi: 10.1063/1.4861427. Full text in Research Archive
• Merlot, Patrick; Izsak, Robert; Borgoo, Alex; Kjærgaard, Thomas; Helgaker, Trygve & Reine, Simen Sommerfelt (2014). Charge-constrained auxiliary-density-matrix methods for the Hartree-Fock exchange contribution. Journal of Chemical Physics. ISSN 0021-9606. 141(9). doi: 10.1063/1.4894267. Full text in Research Archive
• Tellgren, Erik; Kvaal, Simen & Helgaker, Trygve (2014). Fermion N -representability for prescribed density and paramagnetic current density. Physical Review A. Atomic, Molecular, and Optical Physics (PRA). ISSN 1050-2947. 89(1). doi: 10.1103/PhysRevA.89.012515. Full text in Research Archive Show summary

  The N-representability problem is the problem of determining whether there exists N-particle states with some prescribed property. Here we report an affirmative solution to the fermion N-representability problem when both the density and the paramagnetic current density are prescribed. This problem arises in current-density functional theory and is a generalization of the well-studied corresponding problem (only the density prescribed) in density functional theory. Given any density and paramagnetic current density satisfying a minimal regularity condition (essentially that a von Weizäcker–like canonical kinetic energy density is locally integrable), we prove that there exists a corresponding N-particle state. We prove this by constructing an explicit one-particle reduced density matrix in the form of a position-space kernel, i.e., a function of two continuous-position variables. In order to make minimal assumptions, we also address mathematical subtleties regarding the diagonal of, and how to rigorously extract paramagnetic current densities from, one-particle reduced density matrices in kernel form. © 2014 American Physical Society

• Ringholm, Magnus; Jonsson, Dan Johan; Bast, Radovan; Gao, Bin; Thorvaldsen, Andreas johan & Ekstrøm, Ulf [Show all 8 contributors for this article] (2014). Analytic cubic and quartic force fields using density-functional theory. Journal of Chemical Physics. ISSN 0021-9606. 140(3). doi: 10.1063/1.4861003. Full text in Research Archive Show summary

  We present the first analytic implementation of cubic and quartic force constants at the level of Kohn–Sham density-functional theory. The implementation is based on an open-ended formalism for the evaluation of energy derivatives in an atomic-orbital basis. The implementation relies on the availability of open-ended codes for evaluation of one- and two-electron integrals differentiated with respect to nuclear displacements as well as automatic differentiation of the exchange–correlation kernels. We use generalized second-order vibrational perturbation theory to calculate the fundamental frequencies of methane, ethane, benzene, and aniline, comparing B3LYP, BLYP, and Hartree–Fock results. The Hartree–Fock anharmonic corrections agree well with the B3LYP corrections when calculated at the B3LYP geometry and from B3LYP normal coordinates, suggesting that the inclusion of electron correlation is not essential for the reliable calculation of cubic and quartic force constants.

• Smalø, Hans Sverre; Rybkin, Vladimir; Klopper, Wim; Helgaker, Trygve & Uggerud, Einar (2014). Mechanochemistry: The effect of dynamics. Journal of Physical Chemistry A. ISSN 1089-5639. 118(36), p. 7683–7694. doi: 10.1021/jp504959z. Full text in Research Archive
• Rebolini, Elisa; Toulouse, Julien; Teale, Andrew Michael; Helgaker, Trygve & Savin, Andreas (2014). Excitation energies along a range-separated adiabatic connection. Journal of Chemical Physics. ISSN 0021-9606. 141(4). doi: 10.1063/1.4890652. Full text in Research Archive
• Aidas, Kestutis; Angeli, Celestino; Bak, Keld L.; Bakken, Vebjørn; Bast, Radovan & Boman, Linus [Show all 84 contributors for this article] (2014). The Dalton quantum chemistry program system. Wiley Interdisciplinary Reviews. Computational Molecular Science. ISSN 1759-0876. 4(3). doi: 10.1002/wcms.1172. Full text in Research Archive Show summary

  Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree–Fock, Kohn–Sham, multiconfigurational self-consistent-field, Møller–Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge-origin-invariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

• Onishi, Taku & Helgaker, Trygve (2013). A Theoretical Study on Hydrogen Transport Mechanism in SrTio(3) Perovskite. International Journal of Quantum Chemistry. ISSN 0020-7608. 112(1), p. 201–207. doi: 10.1002/qua.23203. Full text in Research Archive
• Rekkedal, Johannes Andreas; Coriani, Sonia; Iozzi, Maria Francesca; Teale, Andrew Michael; Helgaker, Trygve & Pedersen, Thomas Bondo (2013). Communication: Analytic gradients in the random-phase approximation. Journal of Chemical Physics. ISSN 0021-9606. 139(8). doi: 10.1063/1.4819399. Full text in Research Archive
• Przybytek, Michal & Helgaker, Trygve (2013). The accuracy of the Gaussian-and-finite-element-Coulomb (GFC) method for the calculation of Coulomb integrals. Journal of Chemical Physics. ISSN 0021-9606. 139(5). doi: 10.1063/1.4817002. Full text in Research Archive
• Stoyanova, Alexandrina; Teale, Andrew Michael; Toulouse, Julien; Helgaker, Trygve & Fromager, Emmanuel (2013). Alternative separation of exchange and correlation energies in multi-configuration range-separated density-functional theory. Journal of Chemical Physics. ISSN 0021-9606. 139(13). doi: 10.1063/1.4822135. Full text in Research Archive
• Onishi, Taku & Helgaker, Trygve (2013). A Theoretical Study on Proton Conduction Mechanism in BaZrO3 Perovskite. In Hotokka, Matti; Brändas, Erkki J.; Maruani, Jean & Delgado-Barrio, Gerardo (Ed.), Advances in Quantum Methods and Applications in Chemistry, Physics, and Biology. Springer. ISSN 9783319015286. p. 238–248. doi: 10.1007/978-3-319-01529-3_14. Full text in Research Archive
• Rybkin, Vladimir; Ekstrøm, Ulf & Helgaker, Trygve (2013). Internal-to-Cartesian Back Transformation of Molecular Geometry Steps Using High-Order Geometric Derivatives. Journal of Computational Chemistry. ISSN 0192-8651. 34, p. 1842–1849. doi: 10.1002/jcc.23327. Full text in Research Archive
• Rybkin, Vladimir; Simakov, Anton; Bakken, Vebjørn; Reine, Simen Sommerfelt; Kjærgaard, Thomas & Helgaker, Trygve [Show all 7 contributors for this article] (2013). Insights into the dynamics of evaporation and proton migration in protonated water clusters from large-scale bornoppenheimer direct dynamics. Journal of Computational Chemistry. ISSN 0192-8651. 34(7), p. 533–544. doi: 10.1002/jcc.23162. Full text in Research Archive
• Helgaker, Trygve; Gauss, Jürgen; Gabriele, Cazzoli & Cristina, Puzzarini (2013). 33S hyperfine interactions in H2S and SO2 and revision of the sulfur nuclear magnetic shielding scale. Journal of Chemical Physics. ISSN 0021-9606. 139(24:244308). doi: 10.1063/1.4849177. Full text in Research Archive
• Onishi, Taku & Helgaker, Trygve (2013). A Theoretical Study on the Hydrogen Transport Mechanism in SrTiO 3 Perovskite. II. Scandium Doping at Titanium Site. International Journal of Quantum Chemistry. ISSN 0020-7608. 113, p. 599–604. doi: 10.1002/qua.24086. Full text in Research Archive
• Merlot, Patrick; Kjærgaard, Thomas; Helgaker, Trygve; Lindh, Roland; Aquilante, Francesco & Reine, Simen Sommerfelt [Show all 7 contributors for this article] (2013). Attractive electron-electron interactions within robust local fitting approximations. Journal of Computational Chemistry. ISSN 0192-8651. 34(17), p. 1486–1496. doi: 10.1002/jcc.23284. Full text in Research Archive Show summary

  An analysis of Dunlap's robust fitting approach reveals that the resulting two-electron integral matrix is not manifestly positive semidefinite when local fitting domains or non-Coulomb fitting metrics are used. We present a highly local approximate method for evaluating four-center two-electron integrals based on the resolution-of-the-identity (RI) approximation and apply it to the construction of the Coulomb and exchange contributions to the Fock matrix. In this pair-atomic resolution-of-the-identity (PARI) approach, atomic-orbital (AO) products are expanded in auxiliary functions centered on the two atoms associated with each product. Numerical tests indicate that in 1% or less of all Hartree-Fock and Kohn-Sham calculations, the indefinite integral matrix causes nonconvergence in the self-consistent-field iterations. In these cases, the two-electron contribution to the total energy becomes negative, meaning that the electronic interaction is effectively attractive, and the total energy is dramatically lower than that obtained with exact integrals. In the vast majority of our test cases, however, the indefiniteness does not interfere with convergence. The total energy accuracy is comparable to that of the standard Coulomb-metric RI method. The speed-up compared with conventional algorithms is similar to the RI method for Coulomb contributions; exchange contributions are accelerated by a factor of up to eight with a triple-zeta quality basis set. A positive semidefinite integral matrix is recovered within PARI by introducing local auxiliary basis functions spanning the full AO product space, as may be achieved by using Cholesky-decomposition techniques. Local completion, however, slows down the algorithm to a level comparable with or below conventional calculations.

• Teale, Andrew Michael; Lutnæs, Ola Berg; Helgaker, Trygve; Tozer, David J. & Gauss, Jürgen (2013). Benchmarking density-functional theory calculations of NMR shielding constants and spin-rotation constants using accurate coupled-cluster calculations. Journal of Chemical Physics. ISSN 0021-9606. 138(2). doi: 10.1063/1.4773016. Full text in Research Archive
• Helgaker, Trygve; Coriani, Sonia; Jørgensen, Poul; Kristensen, Kasper; Olsen, Jeppe & Ruud, Kenneth (2012). Recent Advances in Wave Function-Based Methods of Molecular-Property Calculations. Chemical Reviews. ISSN 0009-2665. 112(1), p. 543–631. doi: 10.1021/cr2002239. Full text in Research Archive
• Kaarvann, Lange Kai; Tellgren, Erik; Hoffmann, Mark & Helgaker, Trygve (2012). A Paramagnetic Bonding Mechanism for Diatomics in Strong Magnetic Fields. Science. ISSN 0036-8075. 337(6092), p. 327–331. doi: 10.1126/science.1219703. Full text in Research Archive
• Reine, Simen Sommerfelt; Helgaker, Trygve & Lindh, Roland (2012). Multi-electron integrals. Wiley Interdisciplinary Reviews. Computational Molecular Science. ISSN 1759-0876. 2(2), p. 290–303. doi: 10.1002/wcms.78. Full text in Research Archive
• Middendorf, Nils; Höfener, Sebastian; Klopper, Wim & Helgaker, Trygve (2012). Calculation of the two-electron Darwin term using explicitly correlated wave functions. Chemical Physics. ISSN 0301-0104. 401, p. 146–151. doi: 10.1016/j.chemphys.2011.10.035. Full text in Research Archive
• Helgaker, Trygve; Jaszunski, Michal; Garbacz, Piotr & Jackowski, Karol (2012). The NMR indirect nuclear spin-spin coupling constant of the HD molecule. Molecular Physics. ISSN 0026-8976. 110(19-20), p. 2611–2617. doi: 10.1080/00268976.2012.729097. Full text in Research Archive
• Tellgren, Erik; Reine, Simen Sommerfelt & Helgaker, Trygve (2012). Analytical GIAO and hybrid-basis integral derivatives: application to geometry optimization of molecules in strong magnetic fields. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 14(26), p. 9492–9499. doi: 10.1039/c2cp40965h. Full text in Research Archive
• Onishi, Taku & Helgaker, Trygve (2012). A Theoretical Study on Hydrogen Transport Mechanism in SrTio(3) Perovskite. International Journal of Quantum Chemistry. ISSN 0020-7608. 112(1), p. 201–207. doi: 10.1002/qua.23203. Full text in Research Archive
• Teale, Andrew Michael; Coriani, Sonia & Helgaker, Trygve (2012). Range-dependent adiabatic connections. AIP Conference Proceedings. ISSN 0094-243X. 1504, p. 92–99. doi: 10.1063/1.4771706. Full text in Research Archive Show summary

  Recently, we have implemented a scheme for the calculation of the adiabatic connection linking the Kohn-Sham system to the physical, interacting system. This scheme uses a generalized Lieb functional, in which the electronic-interaction strength is varied in a simple linear fashion, keeping the potential or the density fixed in the process. In the present work, we generalize this scheme further to accommodate arbitrary two-electron operators, allowing the calculation of adiabatic connections following alternative paths as outlined by Yang [J. Chem. Phys. 109, 10107 (1998)]. Specifically, we examine the error-function and Gaussian-attenuated error-function adiabatic connections. We explore the high-density and strong static-correlation regimes for two-electron systems. The resulting adiabatic connections give an alternative view of the exchange- correlation problem and their utility for the development of new exchange-correlation functionals in Kohn-Sham and rangeseparated hybrid schemes is discussed. This research was originally published in AIP Conference Proceedings. © AIP Publishing

• Tellgren, Erik; Kvaal, Simen; Sagvolden, Espen; Ekström, Ulf Egil; Teale, Andrew Michael & Helgaker, Trygve (2012). Choice of basic variables in current-density-functional theory. Physical Review A. Atomic, Molecular, and Optical Physics (PRA). ISSN 1050-2947. 86(6). doi: 10.1103/PhysRevA.86.062506. Full text in Research Archive Show summary

  The selection of basic variables in current-density-functional theory and formal properties of the resulting formulations are critically examined. Focus is placed on the extent to which the Hohenberg-Kohn theorem, constrained-search approach, and Lieb's formulation (in terms of convex and concave conjugation) of standard density-functional theory can be generalized to provide foundations for current-density-functional theory. For the well-known case with the gauge-dependent paramagnetic current density as a basic variable, we find that the resulting total energy functional is not concave. It is shown that a simple redefinition of the scalar potential restores concavity and enables the application of convex analysis and convex (or concave) conjugation. As a result, the solution sets arising in potential-optimization problems can be given a simple characterization. We also review attempts to establish theories with the physical current density as a basic variable. Despite the appealing physical motivation behind this choice of basic variables, we find that the mathematical foundations of the theories proposed to date are unsatisfactory. Moreover, the analogy to standard density-functional theory is substantially weaker as neither the constrained-search approach nor the convex analysis framework carry over to a theory making use of the physical current density. © 2012 American Physical Society

• Iozzi, Maria Francesca; Helgaker, Trygve & Uggerud, Einar (2011). Influence of External Force on Properties and Reactivity of Disulfide Bonds. Journal of Physical Chemistry A. ISSN 1089-5639. 115(11), p. 2308–2315. doi: 10.1021/jp109428g. Full text in Research Archive
• Bast, Radovan; Ruud, Kenneth; Rizzo, Antonio & Helgaker, Trygve (2011). Relativistic four-component calculations of Buckingham birefringence using London atomic orbitals. Theoretical Chemistry Accounts. ISSN 1432-881X. 129(3-5), p. 685–699. doi: 10.1007/s00214-011-0939-3. Full text in Research Archive Show summary

  We present the first relativistic study of the electric-field-gradient induced birefringence (Buckingham birefringence), with application to the series of molecules CX2 (X = O, S, Se, Te). A recently developed atomic-orbital-driven scheme for the calculation of time-dependent molecular properties using one-, two- and four-component relativistic wave functions (Bast et al. in Chem Phys 356:177, 2009) is extended to first-order frequency-dependent magnetic-field perturbations, using London atomic orbitals to ensure gauge-origin independent results and to improve basis-set convergence. Calculations are presented at the Hartree–Fock and Kohn–Sham levels of theory and results for CO2 and CS2 are compared with previous high-level coupled-cluster calculations. Except for the heaviest member of the series, relativistic effects are small—in particular for the temperature-independent contribution to the birefringence. By contrast, the effects of electron correlation are significant. However, the reliability of standard exchange-correlation functionals in describing Buckingham birefringence remains unclear based on the comparison with high-level coupled-cluster singles-and-doubles calculations.

• Klopper, Wim; Teale, Andrew Michael; Coriani, Sonia; Pedersen, Thomas Bondo & Helgaker, Trygve (2011). Spin flipping in ring-coupled-cluster-doubles theory. Chemical Physics Letters. ISSN 0009-2614. 510(1-3), p. 147–153. doi: 10.1016/j.cplett.2011.04.101. Full text in Research Archive
• Strømsheim, Marie Døvre; Kumar, Neena Theresa; Coriani, Sonia; Sagvolden, Espen; Teale, Andrew Michael & Helgaker, Trygve (2011). Dispersion interactions in density-functional theory: An adiabatic-connection analysis. Journal of Chemical Physics. ISSN 0021-9606. 135(19). doi: 10.1063/1.3660357. Full text in Research Archive
• Bast, Radovan; Ulf, Ekstrom; Gao, Bin; Helgaker, Trygve; Ruud, Kenneth & Thorvaldsen, Andreas johan (2011). The ab initio calculation of molecular electric, magnetic and geometric properties. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 13(7), p. 2627–2651. doi: 10.1039/c0cp01647k. Full text in Research Archive
• Lutnæs, Ola Berg; Helgaker, Trygve & Jaszunski, Michal (2010). Spin-spin coupling constants and triplet instabilities in Kohn-Sham theory. Molecular Physics. ISSN 0026-8976. 108(19-20), p. 2579–2590. doi: 10.1080/00268976.2010.513344. Full text in Research Archive
• Teale, Andrew Michael; Coriani, Sonia & Helgaker, Trygve (2010). Range-dependent adiabatic connections. Journal of Chemical Physics. ISSN 0021-9606. 133(16). doi: 10.1063/1.3488100. Full text in Research Archive
• Teale, Andrew Michael; Coriani, Sonia & Helgaker, Trygve (2010). Accurate calculation and modeling of the adiabatic connection in density functional theory. Journal of Chemical Physics. ISSN 0021-9606. 132(16). doi: 10.1063/1.3380834. Full text in Research Archive
• Reine, Simen Sommerfelt; Krapp, Andreas; Iozzi, Maria Francesca; Bakken, Vebjørn; Helgaker, Trygve & Pawlowski, Filip [Show all 7 contributors for this article] (2010). An efficient density-functional-theory force evaluation for large molecular systems. Journal of Chemical Physics. ISSN 0021-9606. 133(4). doi: 10.1063/1.3459061. Full text in Research Archive
• Jansik, Branislav; Høst, Stinne; Johansson, M. P.; Olsen, Jeppe; Jørgensen, Poul & Helgaker, Trygve (2009). Robust and Reliable Multilevel Minimization of the Kohn-Sham Energy. Journal of Chemical Theory and Computation. ISSN 1549-9618. 5(4), p. 1027–1032. doi: 10.1021/ct800562h. Full text in Research Archive
• Tellgren, Erik; Helgaker, Trygve & Soncini, Alessandro (2009). Non-perturbative magnetic phenomena in closed-shell paramagnetic molecules. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 11(26), p. 5489–5498. doi: 10.1039/b822262b. Full text in Research Archive
• Friedrich, Joachim; Coriani, Sonia; Helgaker, Trygve & Dolg, Michael (2009). Implementation of the incremental scheme for one-electron first-order properties in coupled-cluster theory. Journal of Chemical Physics. ISSN 0021-9606. 131(15). doi: 10.1063/1.3243864. Full text in Research Archive
• Jansik, B; Host, S; Johansson, MP; Olsen, J; Jorgensen, P & Helgaker, Trygve (2009). A stepwise atomic, valence-molecular, and full-molecular optimisation of the Hartree-Fock/Kohn-Sham energy. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 11(27), p. 5805–5813. doi: 10.1039/b901987a. Full text in Research Archive
• Teale, Andrew Michael; Coriani, Sonia & Helgaker, Trygve (2009). The calculation of adiabatic-connection curves from full configuration-interaction densities: Two-electron systems. Journal of Chemical Physics. ISSN 0021-9606. 130(10). doi: 10.1063/1.3082285. Full text in Research Archive
• Hofener, S; Tew, DP; Klopper, W & Helgaker, Trygve (2009). The geminal basis in explicitly correlated wave functions. Chemical Physics. ISSN 0301-0104. 356(1-3), p. 25–30. doi: 10.1016/j.chemphys.2008.10.022. Full text in Research Archive
• Lutnæs, Ola B.; Teale, AM; Helgaker, Trygve; Tozer, DJ; Ruud, Kenneth & Gauss, J (2009). Benchmarking density-functional-theory calculations of rotational g tensors and magnetizabilities using accurate coupled-cluster calculations. Journal of Chemical Physics. ISSN 0021-9606. 131(14). doi: 10.1063/1.3242081. Full text in Research Archive
• Iozzi, Maria Francesca; Helgaker, Trygve & Uggerud, Einar (2009). Assessment of theoretical methods for the determination of the mechanochemical strength of covalent bonds. Molecular Physics. ISSN 0026-8976. 107(23-24), p. 2537–2546. doi: 10.1080/00268970903401041. Full text in Research Archive
• Bast, Radovan; Hesselmann, Andreas; Salek, Pawel; Helgaker, Trygve & Saue, Trond (2008). Static and frequency-dependent dipole-dipole polarizabilities of all closed-shell atoms up to radium: a four-component relativistic density functional theory study. ChemPhysChem. ISSN 1439-4235. 9, p. 445–445. Full text in Research Archive
• Host, S; Jansik, B; Olsen, J; Jorgensen, P; Reine, Simen Sommerfelt & Helgaker, Trygve (2008). A ground-state-directed optimization scheme for the Kohn-Sham energy. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 10, p. 5344–5348. doi: 10.1039/b807639a. Full text in Research Archive
• Dahle, Pål; Helgaker, T; Jonsson, Dan Johan & Taylor, PR (2008). Second-order Moller-Plesset calculations on the water molecule using Gaussian-type orbital and Gaussian-type geminal theory. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 10(23), p. 3377–3382. doi: 10.1039/b803577f. Full text in Research Archive Show summary

  The Gaussian-type orbital and Gaussian-type geminal (GGn) model is applied to the water molecule, at the level of second-order Møller–Plesset (MP2) theory. In GGn theory, correlation factors are attached to all doubly-occupied orbital pairs (GG0), to all doubly-occupied and singly-excited pairs (GG1), or to all orbital pairs (GG2). Optimizing the GG2 model using a weak-orthogonality functional, we obtain the current best estimate of the all-electron MP2 correlation energy of water, −361.95 mEh. In agreement with previous observations, the GG1 model performs almost as well as the GG2 model (−361.26 mEh), whereas the GG0 model is poorer (−351.36 mEh). For the barrier to linearity of water, we obtain an MP2 correlation contribution of −463 ± 5 cm−1.

• Kiaergaard, T; Jorgensen, P; Olsen, J; Coriani, Sonia & Helgaker, Trygve (2008). Hartree-Fock and Kohn-Sham time-dependent response theory in a second-quantization atomic-orbital formalism suitable for linear scaling. Journal of Chemical Physics. ISSN 0021-9606. 129. doi: 10.1063/1.2961039. Full text in Research Archive
• Lutnæs, Ola Berg; Helgaker, Trygve & Jaszunski, M (2008). Density-functional calculations of the nuclear magnetic shielding and indirect nuclear spin-spin coupling constants of three isomers of C20. Molecular Physics. ISSN 0026-8976. 106(20), p. 2357–2365. doi: 10.1080/00268970802467994. Full text in Research Archive
• Soncini, Alessandro; Teale, Andrew Michael; Helgaker, Trygve; Proft, F De & Tozer, DJ (2008). Maps of current density using density-functional methods. Journal of Chemical Physics. ISSN 0021-9606. 129. doi: 10.1063/1.2969104. Full text in Research Archive
• Reine, Simen Sommerfelt; Tellgren, Erik; Krapp, Andreas; Kjærgaard, Thomas; Helgaker, Trygve & Jansik, Branislav [Show all 8 contributors for this article] (2008). Variational and robust density fitting of four-center two-electron integrals in local metrics. Journal of Chemical Physics. ISSN 0021-9606. 129(10). doi: 10.1063/1.2956507. Full text in Research Archive Show summary

  Density fitting is an important method for speeding up quantum-chemical calculations. Linear-scaling developments in Hartree-Fock and density-functional theories have highlighted the need for linear-scaling density-fitting schemes. In this paper, we present a robust variational density-fitting scheme that allows for solving the fitting equations in local metrics instead of the traditional Coulomb metric, as required for linear scaling. Results of fitting four-center two-electron integrals in the overlap and the attenuated Gaussian damped Coulomb metric are presented, and we conclude that density fitting can be performed in local metrics at little loss of chemical accuracy. We further propose to use this theory in linear-scaling density-fitting developments.

• Helgaker, Trygve; Klopper, Wim & Tew, David P. (2008). Quantitative quantum chemistry. Molecular Physics. ISSN 0026-8976. 106, p. 2107–2143. Full text in Research Archive
• Helgaker, Trygve; Jaszunski, Michal & Pecul, Magdalena (2008). The quantum-chemical calculation of NMR indirect spin-spin coupling constants. Progress in nuclear magnetic resonance spectroscopy. ISSN 0079-6565. 53, p. 249–268. Full text in Research Archive
• Kristensen, Kasper; Jørgensen, Poul; Thorvaldsen, Andreas johan & Helgaker, Trygve (2008). Efficient elimination of response parameters in molecular property calculations for variational and nonvariational energies. Journal of Chemical Physics. ISSN 0021-9606. 129(21). doi: 10.1063/1.3023123. Full text in Research Archive
• Høst, Stinne; Olsen, Jeppe; Branislav, Jansik; Thøgersen, Lea; Jørgensen, Poul & Helgaker, Trygve (2008). The augmented Roothaan-Hall method for optimizing Hartree-Fock and Kohn-Sham density matrices. Journal of Chemical Physics. ISSN 0021-9606. 129(12). Full text in Research Archive Show summary

  We present a novel method for the optimization of Hartree-Fock and Kohn-Sham energies that does not suffer from the flaws of the conventionally used two-step Roothaan-Hall (RH) with direct inversion in iterative subspace (DIIS) acceleration scheme, improving the reliability of the optimization while reducing its cost. The key to its success is the replacement of the two separate steps of each RH/DIIS iteration by a single concerted step that fully exploits the Hessian information available from the previous iterations. It is a trust-region based method and therefore by design converges to an energy minimum. Numerical examples are given to illustrate that the algorithm is robust and cost efficient, converging smoothly to a minimum also in cases when the RH/DIIS algorithm fails to converge or when it converges to a saddle point rather than to a minimum. The algorithm is based on matrix multiplications and becomes linearly scaling for sufficiently large systems.

• Peach, Michael J. G.; Benfield, Peter; Helgaker, Trygve & Tozer, David J. (2008). Excitation energies in density functional theory: An evaluation and a diagnostic test. Journal of Chemical Physics. ISSN 0021-9606. 128. doi: 10.1063/1.2831900. Full text in Research Archive
• Bast, Radovan; Hesselmann, Andreas; Salek, Pawel; Helgaker, Trygve & Saue, Trond (2008). Static and Frequency-Dependent Dipole–Dipole Polarizabilities of All Closed-Shell Atoms up to Radium: A Four-Component Relativistic DFT Study. ChemPhysChem. ISSN 1439-4235. 9(3), p. 445–53. doi: 10.1002/cphc.200700504. Full text in Research Archive
• Tellgren, Erik I; Soncini, Alessandro & Helgaker, Trygve (2008). Nonperturbative ab initio calculations in strong magnetic fields using London orbitals. Journal of Chemical Physics. ISSN 0021-9606. 129, p. 154114-1–154114-9. Full text in Research Archive
• Dahle, Pål; Helgaker, Trygve; Jonsson, Dan & Taylor, Peter R. (2007). Explicit Electron Correlation by a Combined Use of Gaussian-Type Orbitals and Gaussian-Type Geminals. In Simos, Theodore E. & Maroulis, George (Ed.), COMPUTATION IN MODERN SCIENCE AND ENGINEERING: Proceedings of the International Conference on Computational Methods in Science and Engineering 2007 (ICCMSE 2007): VOLUME 2, PARTS A and B. AIP Publishing (American Institute of Physics). ISSN 9780735404786. p. 187–190. Full text in Research Archive
• Coriani, Sonia; Høst, Stinne; Jansik, Branislav; Thøgersen, Lea; Olsen, Jeppe & Jørgensen, Poul [Show all 10 contributors for this article] (2007). Linear-scaling implementation of molecular response theory in self-consistent field electronic-structure theory. Journal of Chemical Physics. ISSN 0021-9606. 126. Full text in Research Archive
• Dahle, Pål; Helgaker, T; Jonsson, D & Taylor, PR (2007). Accurate quantum-chemical calculations using Gaussian-type geminal and Gaussian-type orbital basis sets: applications to atoms and diatomics. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 9. doi: 10.1039/b616488a. Full text in Research Archive
• Skomorowski, Wojciech; Pecul, Magdalena; Salek, Pawel & Helgaker, Trygve (2007). Electronic circular dichroism of disulphide bridge: Ab initio quantum-chemical calculations. Journal of Chemical Physics. ISSN 0021-9606. 127. doi: 10.1063/1.2759203. Full text in Research Archive
• Reine, Simen Sommerfelt; Tellgren, Erik & Helgaker, Trygve (2007). A unified scheme for the calculation of differentiated and undifferentiated molecular integrals over solid-harmonic Gaussians. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 9, p. 4771–4779. doi: 10.1039/b705594c. Full text in Research Archive
• Peach, Michael J. G.; Tellgren, Erik; Salek, Pawel; Helgaker, Trygve & Tozer, David J. (2007). Structural and Electronic Properties of Polyacetylene and Polyyne from Hybrid and Coulomb-Attenuated Density Functionals. Journal of Physical Chemistry A. ISSN 1089-5639. 111, p. 11930–11935. doi: 10.1021/jp0754839. Full text in Research Archive
• Tew, David P.; Klopper, Wim & Helgaker, Trygve (2007). Electron correlation: the many-body problem at the heart of chemistry. Journal of Computational Chemistry. ISSN 0192-8651. 28, p. 1307–1320. Full text in Research Archive
• Salek, Pawel; Høst, Stinne; Thøgersen, Lea; Jørgensen, Poul; Manninen, Pekka & Olsen, Jeppe [Show all 12 contributors for this article] (2007). Linear-scaling implementation of molecular electronic self-consistent field theory. Journal of Chemical Physics. ISSN 0021-9606. 126. Full text in Research Archive
• Helgaker, Trygve; Lutnæs, Ola Berg & Jaszunski, Michal (2007). Density-functional and coupled-cluster singles-and-doubles calculations of the nuclear shielding and indirect nuclear spin-spin coupling constants of o-benzyne. Journal of Chemical Theory and Computation. ISSN 1549-9618. 3, p. 86–94. doi: 10.1021/ct600234n. Full text in Research Archive
• Jansik, Branislav; Høst, Stinne; Jørgensen, Poul; Olsen, Jeppe & Helgaker, Trygve (2007). Linear-scaling symmetric square-root decomposition of the overlap matrix. Journal of Chemical Physics. ISSN 0021-9606. 126. Full text in Research Archive
• Keal, Thomas W.; Helgaker, Trygve; Salek, Pawel & Tozer, David J. (2006). Choice of exchange-correlation functional for computing NMR indirect spin-spin coupling constants. Chemical Physics Letters. ISSN 0009-2614. 425, p. 163–166. doi: 10.1016/j.cplett.2006.05.032. Full text in Research Archive
• Wilson, David John; Helgaker, Trygve & Rizzo, Antonio (2006). The magnetizability, rotational g tensor and quadrupole moment of the boron trihalides. Molecular Physics. ISSN 0026-8976. 104. Full text in Research Archive
• Helgaker, Trygve; Hennum, Alf Christian & Klopper, Wim (2006). A second-quantization framework for the unified treatment of relativistic and nonrelativistic molecular perturbations by response theory. Journal of Chemical Physics. ISSN 0021-9606. 125. doi: 10.1063/1.2198527. Full text in Research Archive
• Høst, Stinne; Olsen, Jeppe; Jansik, Branislav; Jørgensen, Poul; Reine, Simen & Helgaker, Trygve [Show all 8 contributors for this article] (2006). Towards black-box linear scaling optimization in Hartree-Fock and Kohn-Sham theories. Lecture Series on Computer and Computational Sciences. ISSN 1573-4196. 6, p. 177–189. Full text in Research Archive
• Lutnæs, Ola Berg; Teale, Andrew M.; Helgaker, Trygve & Tozer, David J. (2006). Rotational g tensors calculated using hybrid exchange-correlation functionals with the optimized effective potential approach. Journal of Chemical Theory and Computation. ISSN 1549-9618. 2, p. 827–834. Full text in Research Archive
• Ferrighi, Lara; Frediani, Luca; Cappelli, Chiara; Salek, Pawel; Ågren, Hans & Helgaker, Trygve [Show all 7 contributors for this article] (2006). Density-functional-theory study of the electric-field-induced second harmonic generation (EFISHG) og push-pull phenylpolyenes in solution. Chemical Physics Letters. ISSN 0009-2614. 425, p. 267–272. Full text in Research Archive
• Paterson, Martin J.; Christiansen, Ove; Pawlowski, Filip Ludwik; Jørgensen, Poul; Hattig, Christof & Helgaker, Trygve [Show all 7 contributors for this article] (2006). Benchmarking two-photon absorption with CC3 quadratic response theory, and comparison with density-functional response theory. Journal of Chemical Physics. ISSN 0021-9606. 124. Full text in Research Archive
• Peach, Michael J. G.; Helgaker, Trygve; Salek, Pawel; Keal, Thomas W.; Lutnæs, Ola Berg & Tozer, David J. [Show all 7 contributors for this article] (2006). Assessment of a Coulomb-attenuated exchange-correlation energy functional. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 8, p. 558–562. doi: 10.1039/b511865d. Full text in Research Archive
• Coriani, Sonia; Haaland, Arne; Helgaker, Trygve & Jørgensen, Poul (2006). The Equilibrium Structure of Ferrocene. ChemPhysChem. ISSN 1439-4235. 7, p. 245–249. doi: 10.1002/cphc.20050033. Full text in Research Archive
• Mohn, Chris E.; Wilson, David J.D.; Lutnæs, Ola B.; Helgaker, Trygve & Ruud, Kenneth (2005). The rotational g tensor as a benchmark for ab initio molecular property calculations. Advances in Quantum Chemistry. ISSN 0065-3276. 50, p. 77–77. Full text in Research Archive
• Thogersen, L; Olsen, J; Kohn, A; Jorgensen, P; Salek, P & Helgaker, Trygve Ulf (2005). The trust-region self-consistent field method in Kohn-Sham density-functional theory. Journal of Chemical Physics. ISSN 0021-9606. 123. Full text in Research Archive
• Salek, P; Helgaker, Trygve Ulf & Saue, T (2005). Linear response at the 4-component relativistic density-functional level: application to the frequency-dependent dipole polarizability of Hg, AuH and PtH2. Chemical Physics. ISSN 0301-0104. 311. Full text in Research Archive
• Rizzo, A; Cappelli, C; Jansik, B; Jonsson, D; Salek, P & Coriani, S [Show all 9 contributors for this article] (2005). Density-functional theory study of electric and magnetic properties of hexafluorobenzene in the vapor phase. Journal of Chemical Physics. ISSN 0021-9606. 122. Full text in Research Archive
• Mohn, Chris Erik; Wilson, David John; Lutnæs, Ola Berg; Helgaker, Trygve & Ruud, Kenneth (2005). The rotational g tensor as a benchmark for ab initio molecular property calculations. Advances in Quantum Chemistry. ISSN 0065-3276. 50, p. 77–90. doi: 10.1016/S0065. Full text in Research Archive
• Salek, Pawel; Helgaker, Trygve; Vahtras, Olav; Ågren, Hans; Jonsson, Dan & Gauss, Jurgen (2005). A comparison of density-functional-theory and coupled-cluster frequency-dependent polarizabilities and hyperpolarizabilities. Molecular Physics. ISSN 0026-8976. 103, p. 439–450. Full text in Research Archive
• Cybulski, H; Pecul, Magdalena; Pecul, M; Helgaker, Trygve & Jaszunski, M (2005). Theoretical studies of nuclear magnetic resonance parameters for the proton-exchange pathways in porphyrin and porphycene. Journal of Physical Chemistry A. ISSN 1089-5639. 109. Full text in Research Archive
• Wilson, David John; Mohn, Chris Erik & Helgaker, Trygve (2005). The Rotational g Tensor as a Benchmark for Density-Functional Theory Calculations of Molecular Magnetic Properties. Journal of Chemical Theory and Computation. ISSN 1549-9618. 123, p. 877–888. doi: 10.1021/ct050101t. Full text in Research Archive
• Olsen, Jeppe; Jørgensen, Poul; Helgaker, Trygve & Oddershede, Jens (2005). Quadratic Response Functions in a Second-Order Polarization Propagator Framework. Journal of Physical Chemistry A. ISSN 1089-5639. 109, p. 11618–11628. doi: 10.1021/jp054207w. Full text in Research Archive
• Sellevåg, Stig Rune; Stenstrøm, Yngve; Helgaker, Trygve & Nielsen, Claus (2005). Atmospheric Chemistry of CHF2CHO: Study of the IR and UV-Vis Absorption Cross Sections, Photolysis, and OH-, Cl-, and NO3-Initiated Oxidation. Journal of Physical Chemistry A. ISSN 1089-5639. 109, p. 3652–3662. doi: 10.1021/jp050313m. Full text in Research Archive
• Coriani, Sonia; Marchesan, Domenico; Gauss, Jurgen; Hattig, Christof; Helgaker, Trygve & Jørgensen, Poul (2005). The accuracy of ab initio molecular geometries for systems containing second-row atoms. Journal of Chemical Physics. ISSN 0021-9606. 123. doi: 10.1063/1.2104387. Full text in Research Archive
• Rudberg, Elias; Salek, Pawel; Helgaker, Trygve & Ågren, Hans (2005). Calculations of two-photon charge-transfer excitations using Coulomb-attenuated density-functional theory. Journal of Chemical Physics. ISSN 0021-9606. 123. doi: 10.1063/1.2104367. Full text in Research Archive
• Rizzo, Antonio; Cappelli, Chiara; Junquera-Hernández, José Miguel; Merás, Alfredo M. J. Sánchez de; Sánchez-Marín, José & Wilson, David John [Show all 7 contributors for this article] (2005). A computational study of some electric and magnetic properties of gaseous BF3 and BCl3. Journal of Chemical Physics. ISSN 0021-9606. 123. doi: 10.1063/1.2034487. Full text in Research Archive
• Bruun, Jesper; Helgaker, Trygve & Jensen, Frank (2004). A comparison of polarization and bond functions for density functional calculations. Molecular Physics. ISSN 0026-8976. 102, p. 2559–2562. Full text in Research Archive
• Bakken, Vebjørn; Helgaker, Trygve & Uggerud, Einar (2004). Models of fragmentations induced by electron attachment to protonated peptides. European Journal of Mass Spectrometry. ISSN 1469-0667. 102, p. 625–638. Full text in Research Archive
• Bakken, Vebjørn & Helgaker, Trygve (2004). The expansion of hydrogen states in Gaussian orbitals. Theoretical Chemistry Accounts. ISSN 1432-881X. 112, p. 124–134. Full text in Research Archive
• Watson, Mark A.; Salek, Pawel Krzysztof; Macak, Peter; Jaszunski, Michal & Helgaker, Trygve (2004). The calculation of indirect nuclear spin-spin coupling constants in large molecules. Chemistry - A European Journal. ISSN 0947-6539. 10, p. 4627–4639. Full text in Research Archive
• Ruden, Torgeir Andersen; Helgaker, Trygve; Jørgensen, Poul & Olsen, Jeppe (2004). Coupled-cluster connected quadruples and quintuples corrections to the harmonic vibrational frequencies and equilibrium bond distances of HF, N-2, F-2, and CO. Journal of Chemical Physics. ISSN 0021-9606. 121, p. 5874–5884. Full text in Research Archive

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• Nordholm, Sture; Baerends, Evert Jan; Burke, Kieron; Gill, Peter; Gori-Giorgi, Paola & Grimme, Stefan [Show all 18 contributors for this article] (2026). The rocky path of DFT into chemistry—Discussions at a symposium and reflections on a circular journey in honor of Axel Becke 1953–2025. Journal of Chemical Physics. ISSN 0021-9606. 165(5). doi: 10.1063/5.0337560. Full text in Research Archive Show summary

  We summarize a recent symposium on density functional theory for electronic structure in chemical applications and honor a key contributor, Axel Becke. The theory of electronic structure is entirely quantum mechanical and, in its original wave function form, runs into mathematical and numerical problems due to high spatial dimensionality, as each electron adds three new coordinates. The approximate reduction of dimensionality in the form of density functional theory (DFT) was first proposed in the mid-1920s but took 40 years to be verified as possible in principle. The development of increasingly accurate functionals, expressing the electronic energy in terms of the density of the electrons, was initially driven by the physics community, but after a series of innovations, including (i) going from local to non-local exchange–correlation energy functionals and (ii) partial retention of exact exchange in terms of wave functions, there was a breakthrough in the early 1990s, and leading quantum chemists accepted DFT. The resulting improved ability to solve chemical problems by computation was rewarded with a Nobel Prize to Walter Kohn and John Pople in 1998. Given the many controversies surrounding the introduction of DFT into chemistry, a symposium was held at the Royal Swedish Academy of Sciences on November 7 and 8, 2024, to look back at the barriers and breakthroughs and identify the key contributors and their advancements. Axel Becke played a major role in bringing DFT into chemistry. We record the unique breadth and depth of the discussions at the symposium and the role of Becke, whose life ended a year thereafter.

• Helgaker, Trygve (2025). A variational reformulation of molecular properties in electronic-structure theory. Full text in Research Archive
• Helgaker, Trygve (2025). The Exotic Chemistry of Ultrastrong Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2025). Egil Hylleraas — Norges første kvantefysiker. Full text in Research Archive
• Helgaker, Trygve (2025). A variational reformulation of molecular properties in electronic-structure theory. Full text in Research Archive
• Helgaker, Trygve (2025). A variational reformulation of molecular properties in electronic-structure theory. Full text in Research Archive
• Helgaker, Trygve (2025). A variational reformulation of molecular properties in electronic-structure theory. Full text in Research Archive
• Helgaker, Trygve (2025). Autobiography of Trygve Helgaker. Journal of Physical Chemistry A. ISSN 1089-5639. 129(50), p. 11471–11479. doi: 10.1021/acs.jpca.5c07793. Full text in Research Archive
• Helgaker, Trygve (2024). My Journey in Theoretical Chemistry — Experiences and Tips. Full text in Research Archive
• Helgaker, Trygve (2024). Minnetale over Arne Haaland. Full text in Research Archive
• Helgaker, Trygve (2024). My Journey in Theoretical Chemisty—Experiences and Tips. Full text in Research Archive
• Helgaker, Trygve (2024). Molecular Dynamics and Rovibrational Spectra in a Magnetic Field. Full text in Research Archive
• Helgaker, Trygve (2024). Hundred Years of Quantum Chemistry in Scandinavia. Full text in Research Archive
• Helgaker, Trygve (2024). High-accuracy quantum chemistry. Full text in Research Archive
• Helgaker, Trygve (2024). Kvantekjemi — på liv og død. Full text in Research Archive
• Helgaker, Trygve (2024). Molecular Bonding, Structure, and Dynamics in a Strong Magnetic Field. Full text in Research Archive
• Helgaker, Trygve (2024). Kvantekjemi. Full text in Research Archive
• Helgaker, Trygve (2023). Kvantekjemi — på liv og død. Full text in Research Archive
• Helgaker, Trygve (2023). Kvantekjemi — på liv og død. Full text in Research Archive
• Helgaker, Trygve (2023). Kvantemekanikk og kjemi. Full text in Research Archive
• Helgaker, Trygve (2023). Kvantekjemi. Full text in Research Archive
• Helgaker, Trygve (2023). Molecular dynamics and rovibrational spectra in a magnetic field. Full text in Research Archive
• Helgaker, Trygve (2023). Kvantekjemi. Full text in Research Archive
• Helgaker, Trygve (2023). Molecular dynamics and rovibrational spectra in a magnetic field. Full text in Research Archive
• Helgaker, Trygve (2022). Kvantemekanikk og kjemi. Full text in Research Archive
• Hansen, Audun Skau; Helgaker, Trygve & Laestadius, Andre (2022). En introduksjon til kvantekjemi / Samarbeid med Talentsenter i realfag. Full text in Research Archive
• Helgaker, Trygve (2022). The Exotic Chemistry in an Ultrastrong Magnetic Field. Full text in Research Archive
• Dembinski, S.; Karwowski, J.; Szudy, J. & Helgaker, Trygve (2022). Foreword. Molecular Physics. ISSN 0026-8976. 120(19-20). doi: 10.1080/00268976.2022.2144060. Full text in Research Archive
• Helgaker, Trygve (2022). Current-Density-Functional Theory in a Convex Treatment. Full text in Research Archive
• Helgaker, Trygve (2022). Molecular Bonding, Structure, and Dynamics in a Strong Magnetic Field. Full text in Research Archive
• Helgaker, Trygve (2022). Molecular Bonding, Structure, and Dynamics in a Strong Magnetic Field. Full text in Research Archive
• Stenseth, Nils Christian; Andreassen, Karin Marie; Danielsen, Anne; Helgaker, Trygve; Jansen, Eystein & Moser, Edvard Ingjald [Show all 11 contributors for this article] (2022). Grunnforskningen er truet. Full text in Research Archive
• Helgaker, Trygve (2022). Kvantekjemi—på liv og død. Full text in Research Archive
• Helgaker, Trygve (2022). Diversity at the Hylleraas Centre. Full text in Research Archive
• Helgaker, Trygve (2021). The Exotic Chemistry in a Strong Magnetic Field. Full text in Research Archive
• Helgaker, Trygve (2021). Molecular Dynamics and Rovibrational Spectra in a Strong Magnetic Field. Full text in Research Archive
• Helgaker, Trygve (2021). The Exotic Chemistry in a Strong Magnetic Field. Full text in Research Archive
• Helgaker, Trygve (2021). The Exotic Chemistry in an Ultrastrong Magnetic Field. Full text in Research Archive
• Helgaker, Trygve (2021). Hylleraas-senteret. Full text in Research Archive
• Helgaker, Trygve (2021). Forskere på sporet av gammelt mysterium: Slik «ser» trekkfugler magnetfelt. [Internet]. NRK. Full text in Research Archive
• Tellgren, Erik & Helgaker, Trygve (2020). Strong Magnetic Field-Induced Chemical Bonding. Full text in Research Archive
• Helgaker, Trygve (2020). Chemistry in a Strong Magnetic Field. Full text in Research Archive
• Eriksen, Janus J.; Stopkowicz, Stella; Jagau, Thomas-C. & Helgaker, Trygve (2020). Foreword: Prof. Gauss Festschrift. Molecular Physics. ISSN 0026-8976. 118. doi: 10.1080/00268976.2020.1817247. Full text in Research Archive
• Helgaker, Trygve (2020). Quantum Chemistry – A Matter of Life and Death. Full text in Research Archive
• Helgaker, Trygve (2020). Hylleraas Centre for Quantum Molecular Sciences – Applying For and Setting Up a Centre of Excellence. Full text in Research Archive
• Helgaker, Trygve (2020). SFF V: Assessment Criteria — Applying For and Setting Up a Centre of Excellence. Full text in Research Archive
• Laestadius, Andre; Penz, Markus; Tellgren, Erik; Ruggenthaler, Michael; Kvaal, Simen & Helgaker, Trygve (2019). Guaranteed convergence of a regularized Kohn-Sham iteration in finite dimensions. Full text in Research Archive Show summary

  Guaranteed convergence of a regularized Kohn-Sham iteration in finite dimensions M. Penz2, A. Laestadius1, E. Tellgren1, M. Ruggenthaler2, S. Kvaal1, T. Helgaker1 1. University of Oslo, Department of Chemistry, Oslo, Germany 2 .Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany The iterative Kohn-Sham scheme [1] has to date not been rigorously shown to converge to the correct ground-state density. This talk addresses the recent result of Penz et al. [2] that demonstrates the convergence of the exact Moreau-Yosida regularized theory in a finite-dimensional setting. This builds on previous work [3], where a similar iterative scheme was proposed that proved a weak type of convergence following an idea by Wagner et al. [4,5]. To obtain the desired convergence in both densities and potentials, the Moreau-Yosida regularization is key for the convergence proof in [2]. This ensures differentiability of the universal Lieb functional [6] and was introduced in density-functional theory (DFT) by Kvaal et al. [7]. It has also recently been successfully applied to paramagnetic current DFT [8]. References [1] W. Kohn and L.J. Sham, Phys. Rev. 140, A1133 (1965). [2] M. Penz, A. Laestadius, E.I. Tellgren, and Michael Ruggenthaler, Phys. Rev. Lett. 123, 037401 (2019). [3] A. Laestadius, M. Penz, E.I. Tellgren, M. Ruggenthaler, S. Kvaal, and T. Helgaker, J. Chem. Phys. 149, 164103 (2018). [4] L.O. Wagner, E. M. Stoudenmire, K. Burke, and S.R. White, Phys. Rev. Lett. 111, 093003 (2013). [5] L.O. Wagner, T. E. Baker, E. M. Stoudenmire, K. Burke, and S.R. White, Physical Review B 90, 045109 (2014). [6] E.H. Lieb, Int. J. Quantum Chem. 24, 243 (1983). [7] S. Kvaal, U. Ekström, A.M. Teale, and T. Helgaker, J. Chem. Phys. 140, 18A518 (2014). [8] A. Laestadius, M. Penz, E.I. Tellgren, M. Ruggenthaler, S. Kvaal, and T. Helgaker, J. Chem. Theory Comput. 15, 4003 (2019).

• Tellgren, Erik; Helgaker, Trygve & Sen, Sangita (2019). Unifying kinetic energy density and current density in density-functional theory. Full text in Research Archive
• Tellgren, Erik; Helgaker, Trygve & Sen, Sangita (2019). Unifying kinetic energy density and current density in density-functional theory. Full text in Research Archive
• Tellgren, Erik; Helgaker, Trygve & Sen, Sangita (2019). Unifying kinetic energy density and current density in density-functional theory. Full text in Research Archive
• Helgaker, Trygve (2019). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2019). Geometry optimization. Full text in Research Archive
• Helgaker, Trygve (2019). Kvantekjemi — på liv og død. Full text in Research Archive
• Helgaker, Trygve (2019). Utenomjordisk kjemi. Full text in Research Archive
• Helgaker, Trygve (2019). Kvantemekanikk og kjemi. Full text in Research Archive
• Helgaker, Trygve (2019). Egil A. Hylleraas – en norsk pioner innen kvantemekanikk. Full text in Research Archive
• Holzer, Christof; Teale, Andrew Michael; Hampe, Florian; Stopkowicz, Stella; Helgaker, Trygve & Klopper, Wim (2019). Erratum: GW quasiparticle energies of atoms in strong magnetic fields (Journal of Chemical Physics (2019) 150 (214112) DOI: 10.1063/1.5093396). Journal of Chemical Physics. ISSN 0021-9606. 151(6). doi: 10.1063/1.5120100. Full text in Research Archive
• Helgaker, Trygve (2019). Robust All-Electron Optimization of Orbital-Free Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve; Ruud, Kenneth & Røsjø, Bjarne (2019). Forskerne ved Hylleraas-senteret jakter på kjemiens hellige gral. [Journal]. Titan.uio.no. Full text in Research Archive
• Helgaker, Trygve (2019). Robust All-Electron Optimization of Orbital-Free Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2019). Chemistry in a strong magnetic field. Full text in Research Archive
• Helgaker, Trygve (2019). Robust All-Electron Optimization of Orbital-Free Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2019). Kvantemekanikk og kjemi. Full text in Research Archive
• Helgaker, Trygve (2019). Kvantemekanikk og kjemi. Full text in Research Archive
• Helgaker, Trygve (2019). Quantum-Chemical Calculations of NMR Parameters. Full text in Research Archive
• Helgaker, Trygve (2019). Molecules in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2019). Hylleraas Centre for Quantum Molecular Sciences: Applying for and Setting up a Centre of Excellence. Full text in Research Archive
• Helgaker, Trygve (2019). Hylleraas Centre for Quantum Molecular Sciences. Full text in Research Archive
• Helgaker, Trygve (2019). Utenomjordisk kjemi. Full text in Research Archive
• Helgaker, Trygve (2019). Centre for Data Science and Computational Science. Full text in Research Archive
• Helgaker, Trygve (2019). Time-independent molecular properties. Full text in Research Archive
• Helgaker, Trygve (2019). Robust All-Electron Optimization of Orbital-Free Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2019). Chemistry in a Strong Magnetic Field. Full text in Research Archive
• Helgaker, Trygve & Isaksen, Karoline Kvellestad (2019). Kan sjaman Durek Verrett snu atomer? – Vi snur atomkjernene hver dag i vitenskap og medisin, svarer forsker. [Journal]. forskning.no. Full text in Research Archive
• Helgaker, Trygve (2019). Egil A. Hylleraas - a pioneer of computational quantum mechanics. Full text in Research Archive
• Helgaker, Trygve; Svelle, Stian & Haugnes, Gunhild Merete (2019). Datakraft gir forskerne bedre intuisjon og flere ideer. [Journal]. Titan.uio.no. Full text in Research Archive
• Laestadius, Andre; Penz, Markus; Tellgren, Erik; Ruggenthaler, Michael; Kvaal, Simen & Helgaker, Trygve (2018). Generalized Kohn-Sham iteration on Banach Spaces. Full text in Research Archive
• Tellgren, Erik; Lange, Kai Kaarvann; Sagvolden, Espen; Kvaal, Simen; Austad, Jon & Reimann, Sarah [Show all 10 contributors for this article] (2018). Quantum chemistry for strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2018). Hylleraas Centre for Quantum Molecular Sciences. Full text in Research Archive
• Helgaker, Trygve (2018). Closed-Shell Molecular Paramagnetism. Full text in Research Archive
• Helgaker, Trygve; Borgoo, Alex; Dimitrova, Maria; Gauss, Jurgen; Hampe, Florian & Holzer, Christof [Show all 13 contributors for this article] (2018). Diamagnetism and Paramagnetism in Atoms and Molecules. Full text in Research Archive
• Helgaker, Trygve (2018). Chemistry in Strong Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2018). Four-Way Correspondence of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2018). Kvantemekanikk og kjemi. Full text in Research Archive
• Helgaker, Trygve; Borgoo, Alex; Gauss, Jurgen; Hampe, Florian; Klopper, Wim & Reimann, Sarah [Show all 9 contributors for this article] (2018). Closed-Shell Molecular Paramagnetism. Full text in Research Archive
• Helgaker, Trygve; Borgoo, Alex; Gauss, Jurgen; Hampe, Florian; Klopper, Wim & Reimann, Sarah [Show all 9 contributors for this article] (2018). Closed-Shell Molecular Paramagnetism. Full text in Research Archive
• Helgaker, Trygve (2018). Utenomjordisk kjemi. Full text in Research Archive
• Helgaker, Trygve (2018). Molecular Derivatives and Properties. Full text in Research Archive
• Helgaker, Trygve; Borgoo, Alex; Gauss, Jurgen; Hampe, Florian; Klopper, Wim & Stopkowicz, Stella [Show all 8 contributors for this article] (2018). Closed-Shell Molecular Magnetism. Full text in Research Archive
• Helgaker, Trygve (2018). Hylleraas Centre for Quantum Molecular Sciences — Ny giv i nytt senter, kvinnelige rollemodeller og nytt initiativ til karrierestøtte, Full text in Research Archive
• Helgaker, Trygve & Ruud, Kenneth (2018). Langsiktig fra Forskningsrådet. Khrono.no. ISSN 1894-8995. Full text in Research Archive
• Helgaker, Trygve & Ruud, Kenneth (2017). 10 år med god kjemi. Full text in Research Archive
• Helgaker, Trygve (2017). Density-Functional Theory in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2017). Density-Functional Theory in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2017). Hylleraas Centre for Quantum Molecular Sciences. Full text in Research Archive
• Pal, Sourav; Helgaker, Trygve & Savin, Andreas (2017). Foreword to special issue in honour of Debashis Mukherjee. Molecular Physics. ISSN 0026-8976. 115(21-22), p. 2627–2628. doi: 10.1080/00268976.2017.1380450. Full text in Research Archive
• Helgaker, Trygve (2017). Four-Way Correspondence of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2017). The Role of the Hohenberg-Kohn Theorem in Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2017). Quantum Chemistry in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2017). The Role of the Hohenberg-Kohn Theorem in Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2017). Principles of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2017). Geometry Optimization. Full text in Research Archive
• Helgaker, Trygve (2017). Hylleraas Centre. Full text in Research Archive
• Lüthi, Hans P. & Helgaker, Trygve (2017). A tribute to Jan Erik Almlof. Molecular Physics. ISSN 0026-8976. 115(17-18), p. 2033–2042. doi: 10.1080/00268976.2017.1293305. Full text in Research Archive
• Helgaker, Trygve (2017). CTCC Review. Full text in Research Archive
• Helgaker, Trygve (2017). Molecules in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2017). Time-Dependent Molecular Properties. Full text in Research Archive
• Helgaker, Trygve (2017). Four-Way Correspondence of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2017). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2016). Derivatives and Properties. Full text in Research Archive
• Borgoo, Alex; Tozer, David J.; Teale, Andrew Michael & Helgaker, Trygve (2016). Approximate orbital-free non-interacting kinetic energy density functionals from density scaling. Full text in Research Archive
• Helgaker, Trygve (2016). Current-Density-Functional Theory of Molecules in Weak and Strong Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2016). The Role of the Hohenberg–Kohn Theorem in Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2016). Recent Developments in Current-Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2016). Molecules in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2016). The Role of the Hohenberg–Kohn Theorem in Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve & Lüthi, Hans Peter (2016). Jan Almlöf. Full text in Research Archive
• Helgaker, Trygve (2016). Current-Density-Functional Theory of Molecules in Weak and Strong Magnetic Fields. Full text in Research Archive
• Rebolini, Elisa; Izsak, Robert; Reine, Simen Sommerfelt; Helgaker, Trygve & Pedersen, Thomas Bondo (2016). Comparison of three efficient approximate exact-exchange algorithms: chain-of-spheres, pair-atomic resolution of the identity, and the auxiliary density matrix method. Full text in Research Archive
• Wirz, Lukas Nico; Reine, Simen Sommerfelt; Pedersen, Thomas Bondo & Helgaker, Trygve (2016). Efficient auxiliary bases for the non-robust pair atomic resolution of the identity (PARI) method. Full text in Research Archive
• Gori-Giorgi, Paola; Helgaker, Trygve; Scuseria, Gustavo E.; Silvi, Bernard & Toulouse, Julien (2016). Foreword for special issue of Molecular Physics in honour of Andreas Savin. Molecular Physics. ISSN 0026-8976. 114, p. 909–909. doi: 10.1080/00268976.2016.1160647. Full text in Research Archive
• Helgaker, Trygve (2015). Molecular magnetic properties. Full text in Research Archive
• Helgaker, Trygve (2015). Geometry optimisation. Full text in Research Archive
• Helgaker, Trygve (2015). Density-functional theory in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2015). Density-functional theory in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2015). Fundamentals of density-functional theory. Full text in Research Archive
• Helgaker, Trygve (2015). Time-Independent molecular properties. Full text in Research Archive
• Helgaker, Trygve & Reine, Simen Sommerfelt (2015). Fremtidens nanomedisiner bygger på kvantekjemi. [Journal]. Apollon. Full text in Research Archive
• Tellgren, Erik; Teale, Andrew Michael; Ekström, Ulf Egil; Kvaal, Simen; Sagvolden, Espen & Helgaker, Trygve (2015). Current density functional theory for molecular systems in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2015). Magnetisk binding. Full text in Research Archive
• Helgaker, Trygve (2015). Density-Functional Theory in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2015). Density-functional theory in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2015). Egil A. Hylleraas. Full text in Research Archive
• Helgaker, Trygve (2015). Exact but Differentiable Formulation of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2015). Moreau–Yosida Regularised DFT. Full text in Research Archive
• Helgaker, Trygve (2015). Egil A. Hylleraas. Full text in Research Archive
• Helgaker, Trygve (2015). Exact but Differentiable Formulation of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve; Knowles, Peter J.; Lee, Timothy J.; RIce, Julia E. & Tozer, David J. (2015). Foreword. Molecular Physics. ISSN 0026-8976. 113, p. 1509–1510. doi: 10.1080/00268976.2015.1047162. Full text in Research Archive
• Borgoo, Alex; Helgaker, Trygve & Tozer, David J. (2015). Exploring homogeneity under density scaling and Molecular binding in post-Kohn–Sham orbital-free DFT. Full text in Research Archive
• Borgoo, Alex; Teale, Andrew Michael & Helgaker, Trygve (2015). Excitation energies from ensemble DFT. Full text in Research Archive
• Borgoo, Alex; Teale, Andrew Michael & Helgaker, Trygve (2015). Excitation energies from ensemble DFT. Full text in Research Archive
• Mennucci, Benedetta; Eisenstein, Odile; Fliegl, Heike; Hopmann, Kathrin Helen; Helgaker, Trygve & Ruud, Kenneth (2015). FemEx—Female Excellence in Theoretical and Computational Chemistry. International Journal of Quantum Chemistry. ISSN 0020-7608. doi: 10.1002/qua.24961. Full text in Research Archive
• Reine, Simen Sommerfelt; Helgaker, Trygve; Merlot, Patrick; Kjærgaard, Thomas; Borgoo, Alex & Izsak, Robert (2014). Charge-constrained auxiliary-density-matrix methods for the Hartree-Fock exchange contribution. Full text in Research Archive
• Sagvolden, Espen; Tellgren, Erik; Ekström, Ulf Egil & Helgaker, Trygve (2014). Cusp based DFT functionals. Full text in Research Archive
• Stopkowicz, Stella & Helgaker, Trygve (2014). Coupled-Cluster Theory for Molecules in Strong Magnetic Fields. Full text in Research Archive Show summary

  In astrochemistry, very strong magnetic fields play an important role. Fields of about 100 T to 100 kT arise around white dwarfs while in neutron stars the fields are even higher (1-100 MT). The interpretation of observed spectra is hampered by the fact that in laboratory measurements magnetic fields can only be produced to up to about 50 T for static fields and to 1000 T in destructive pulse experiments. Therefore, theoretical investigations may help in understanding the substantial influence of strong magnetic fields on the energies and properties of molecules. Usually, magnetic field-de- pendence in quantum-chemical calculations is treated via a perturbative approach using a Taylor expansion around the field B = 0. Employing a more rigorous treatment, however, enables the study of molecules in finite fields of arbitrary strength and led to findings showing distinctly mod- ified chemical properties in very strong fields such as a paramagnetic-to-diamagnetic transition for closed-shell molecules [1] and a new paramagnetic bonding mechanism in diatomics [2]. In this work, coupled-cluster capabilities are introduced to the LONDON program which treats the magnetic field in a non-perturbative manner and uses London orbitals (GIAOs) in order to ensure gauge-origin invariance. As Full-CI results indicate an increasing single-reference character for higher field-strengths, the performance of truncated coupled-cluster approaches is expected to be advantageous. We will present results for energies and magnetizabilities, thereby investigating the influence of field-strengths, electron-correlation, and the use of GIAOs. [1] E. I. Tellgren, T. Helgaker, and A. Soncini, Phys. Chem. Chem. Phys., 11, 5489, (2009). [2] K. K. Lange, E. I. Tellgren, M. R. Hoffmann, and T. Helgaker, Science, 337, 327, (2012).

• Helgaker, Trygve (2014). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2014). Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2014). Basis Sets for Electron Correlation. Full text in Research Archive
• Helgaker, Trygve (2014). Basis Functions and Basis Sets. Full text in Research Archive
• Helgaker, Trygve (2014). Time-Independent Molecular Properties. Full text in Research Archive
• Helgaker, Trygve (2014). Moelcular Integral Evaluation. Full text in Research Archive
• Helgaker, Trygve (2014). The Molecular Hamiltonian. Full text in Research Archive
• Borgoo, Alex; Teale, Andrew Michael & Helgaker, Trygve (2014). Ensemble energies: An alternative route to excitation energies. Full text in Research Archive
• Teale, Andrew Michael; Furness, James; Tellgren, Erik; Ekström, Ulf Egil & Helgaker, Trygve (2014). Density Functional Theory for Molecules in Magnetic Fields. Full text in Research Archive
• Pedersen, Thomas Bondo; Rekkedal, Johannes Andreas; Leikanger, Karl R.; Iozzi, Maria Francesca; Coriani, Sonia & Teale, Andrew Michael [Show all 7 contributors for this article] (2014). Molecular Properties in the Random Phase Approximation. Full text in Research Archive
• Helgaker, Trygve; Kvaal, Simen; Ekström, Ulf Egil & Teale, Andy (2014). Differentiable but Exact Formulation of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2014). Nobelprisen i kjemi 2013 og noe annet... Full text in Research Archive
• Helgaker, Trygve; Kvaal, Simen; Teale, Andrew Michael & Ekström, Ulf Egil (2014). Differentiable but Exact Formulation of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve; Hoffmann, Mark; Lange, Kai Kaarvann; Soncini, Alessandro & Tellgren, Erik (2014). Molecules in Strong Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve; Kvaal, Simen; Ekström, Ulf Egil & Teale, Andrew Michael (2014). Differentiable but Exact Formulation of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve (2014). Beregningskjemi. Full text in Research Archive
• Helgaker, Trygve; Kvaal, Simen; Teale, Andrew Michael; Ekström, Ulf Egil; Jørgensen, Poul & Olsen, Jeppe (2014). Differentiable but Exact Formulation of Density-Functional Theory. Full text in Research Archive
• Helgaker, Trygve; Borgoo, Alex; Ekström, Ulf Egil; Furness, James; Hoffmann, Mark & Kvaal, Simen [Show all 12 contributors for this article] (2014). Density-Functional Theory in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2014). Quantum-Chemical Calculations of NMR Parameters. Full text in Research Archive
• Stopkowicz, Stella & Helgaker, Trygve (2013). Correlated molecular energies in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2013). Density-functional theory, with and without a magnetic field. Full text in Research Archive
• Helgaker, Trygve (2013). Molecular magnetism and density-functional theory in magnetic fields. Full text in Research Archive
• Borgoo, Alex; Savin, A; Knecht, Stefan; Teale, Andrew Michael; Helgaker, Trygve & Sagvolden, Espen (2013). Adiabatic connection for ensemble energies: \\An alternative route to excitation energies. Full text in Research Archive
• Kvaal, Simen; Ekström, Ulf Egil; Tellgren, Erik; Borgoo, Alex; Helgaker, Trygve & Sagvolden, Espen (2013). Moreau-Yosida regularization of DFT. Full text in Research Archive
• Borgoo, Alex; Sagvolden, Espen; Teale, Andrew Michael; Knecht, Stefan; Savin, A & Helgaker, Trygve (2013). Adiabatic connection for ensemble energies: \\An alternative route to excitation energies. Full text in Research Archive
• Helgaker, Trygve (2013). Metoder i teoretisk og beregningsorientert kjemi / Kvantekjemiske beregninger i moderne kjemisk forskning, Full text in Research Archive
• Helgaker, Trygve (2013). Molecules in Strong Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2013). Molecular Magnetic Properties. Full text in Research Archive
• Pedersen, Thomas Bondo; Rekkedal, Johannes Andreas; Iozzi, Maria Francesca; Coriani, Sonia; Teale, Andrew Michael & Helgaker, Trygve (2013). Molecular Properties in the Random Phase Approximation. Full text in Research Archive
• Sagvolden, Espen; Tellgren, Erik; Kvaal, Simen; Ekström, Ulf Egil; Teale, Andrew Michael & Helgaker, Trygve (2013). Building blocks of Current Density Functional Theory. Full text in Research Archive
• Sagvolden, Espen; Tellgren, Erik; Kvaal, Simen; Ekström, Ulf Egil; Teale, Andrew Michael & Helgaker, Trygve (2013). Building blocks of Current Density Functional Theory. Full text in Research Archive
• Ekström, Ulf Egil; Kvaal, Simen; Borgoo, Alex; Helgaker, Trygve; Sagvolden, Espen & Tellgren, Erik (2013). Moreau-Yosida regularization of DFT. Full text in Research Archive
• Helgaker, Trygve (2013). Nobel Prize 2013. Full text in Research Archive
• Helgaker, Trygve (2013). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2013). Molecular magnetism and density-functional theory in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2013). Molecules in Strong Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2013). Molecular Magnetism and Density-Functional Theory in Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2013). Molecular magnetic properties. Full text in Research Archive
• Helgaker, Trygve (2013). TIme-independet molecular properties. Full text in Research Archive
• Helgaker, Trygve (2013). Geometry optimization. Full text in Research Archive
• Helgaker, Trygve (2013). Molecular magnetism and density-functional theory in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). Density-functional theory. Full text in Research Archive
• Helgaker, Trygve (2012). Basis functions and basis sets. Full text in Research Archive
• Helgaker, Trygve (2012). Chemical bonding in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). Basis sets for electron correlation. Full text in Research Archive
• Helgaker, Trygve (2012). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). Time-independent molecular properties. Full text in Research Archive
• Helgaker, Trygve (2012). Molecular integral evaluation. Full text in Research Archive
• Helgaker, Trygve (2012). Time-independent molecular properties. Full text in Research Archive
• Helgaker, Trygve (2012). Current-density-functional theory: basic variables and convex conjugation. Full text in Research Archive
• Helgaker, Trygve (2012). Magnetic resonance parameteres. Full text in Research Archive
• Helgaker, Trygve (2012). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). Molecular bonding in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). Density-functional theory in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). Density-functional theory in magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). Basis functions and basis sets. Full text in Research Archive
• Helgaker, Trygve (2012). Basis sets for electron correlation. Full text in Research Archive
• Helgaker, Trygve (2012). Geometry optimization. Full text in Research Archive
• Gauss, Jurgen; Helgaker, Trygve & Stanton, John F. (2012). Foreword. Molecular Physics. ISSN 0026-8976. 110, p. 2313–2313. doi: 10.1080/00268976.2012.731766. Full text in Research Archive
• Helgaker, Trygve (2012). High-accuracy quantum chemistry. Full text in Research Archive
• Helgaker, Trygve (2012). Chemical bonding in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2012). The molecular Hamiltonian. Full text in Research Archive
• Helgaker, Trygve (2012). The electronic Hamiltonian. Full text in Research Archive
• Helgaker, Trygve (2012). Molecules in an external magnetic field. Full text in Research Archive
• Helgaker, Trygve (2012). Kvantekmekanikk på datamaskiner: kjemiens nye verktøy. Full text in Research Archive
• Helgaker, Trygve (2012). Molecular magnetic propeties. Full text in Research Archive
• Helgaker, Trygve (2011). Importing ab-initio theory into DFT. some applications of the Lieb variation principle. Full text in Research Archive
• Helgaker, Trygve (2011). Kvantemekanikk på datamaskiner: kjemiens nye verktøy. Full text in Research Archive
• Helgaker, Trygve (2011). Centre for Theoretical and Computational Chemistry. Full text in Research Archive
• Helgaker, Trygve (2011). Kvantemekanikk på datamaskiner: kjemiens nye verktøy. Full text in Research Archive
• Helgaker, Trygve (2011). Kvantemekanikk på datamaskiner: kjemiens nye verktøy. Full text in Research Archive
• Helgaker, Trygve (2011). Quantum chemistry: wave-function and density-functional methods. Full text in Research Archive
• Helgaker, Trygve (2011). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2011). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2011). Molecular Electronic-Structure Theory: Yesterday, Today and Tomorrow. Full text in Research Archive
• Helgaker, Trygve (2011). Molecules in strong magnetic fields: electron correlation. Full text in Research Archive
• Helgaker, Trygve (2011). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve; Kaarvann, Lange Kai & Tellgren, Erik (2011). Molecules in strong magnetic fields. META. ISSN 1890-1956. p. 16–18. Full text in Research Archive
• Helgaker, Trygve (2011). TIme-independent molecular properties. Full text in Research Archive
• Helgaker, Trygve (2011). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2011). Molecules in strong magnetic fields: electron correlation. Full text in Research Archive
• Helgaker, Trygve (2011). Molecular electronic structure in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2011). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2011). Geometry Optimization. Full text in Research Archive
• Helgaker, Trygve (2011). Molecular bonding in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2011). Molecular electronic-structure theory; some current trends. Full text in Research Archive
• Helgaker, Trygve (2010). Ab-initio studies of the adiabatic connection in density-functional teory. Full text in Research Archive
• Helgaker, Trygve (2010). Convergence properties of the coupled-cluster method: the accurate calculation of molecular properties for light systems. Full text in Research Archive
• Helgaker, Trygve (2010). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2010). Geometry optimization. Full text in Research Archive
• Helgaker, Trygve (2010). Computational Chemistry. Full text in Research Archive
• Helgaker, Trygve (2010). The calculation of the universal density functional by Lieb maximization. Full text in Research Archive
• Helgaker, Trygve (2010). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2010). Time-Independent Molecular Properties. Full text in Research Archive
• Helgaker, Trygve (2010). The Molecular Hamiltonian. Full text in Research Archive
• Helgaker, Trygve (2010). Basis Functions and Basis Sets. Full text in Research Archive
• Helgaker, Trygve (2010). Basis Sets for Electron Correlation. Full text in Research Archive
• Helgaker, Trygve (2010). The exhaustion of the Schrodinger equation: The accurate calculation of molecular properties of light systems. Full text in Research Archive
• Helgaker, Trygve (2010). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2010). Rigorous calculation of the universal density functional by the Lieb variation principle. Full text in Research Archive
• Helgaker, Trygve (2010). The rigorous calculation of the universal density functional by the Lieb variation principle. Full text in Research Archive
• Helgaker, Trygve (2010). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2010). Ab initio studies for the adiabatic connection in density-functional theory. Full text in Research Archive
• Helgaker, Trygve (2010). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2010). Molecular Integral Evaluation. Full text in Research Archive
• Helgaker, Trygve (2010). Density-functional theory. Full text in Research Archive
• Helgaker, Trygve (2010). Calculation and Modeling of Adiabatic-Connection Curves. Full text in Research Archive
• Helgaker, Trygve (2009). Computational Chemistry. Full text in Research Archive
• Helgaker, Trygve (2009). From ab initio methods to density-functional theory. Full text in Research Archive
• Helgaker, Trygve (2009). Time-Independent Molecular Properties. Full text in Research Archive
• Helgaker, Trygve (2009). The Adiabatic Connection. Full text in Research Archive
• Helgaker, Trygve (2009). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2009). Geometry Optimization. Full text in Research Archive
• Helgaker, Trygve (2009). Linear-Scaling Methods for Molecular Energies and Properties. Full text in Research Archive
• Helgaker, Trygve (2009). The Generalized Adiabatic Connection. Full text in Research Archive
• Helgaker, Trygve (2008). The accurate calculation of molecular energies and properties: a tour of high-accuracy quantum-chemical mthods. Full text in Research Archive
• Helgaker, Trygve (2008). The application of quantum chemistry to large systems: new techniques of self-consistent field theory. Full text in Research Archive
• Helgaker, Trygve (2008). The application of quantum chemistry to large systems: new techniques of self-consistent field theory. Full text in Research Archive
• Helgaker, Trygve (2008). The accurate calculation of molecular energies and properties: A tour of high-accuracy quantum-chemical methods. Full text in Research Archive
• Helgaker, Trygve (2008). The application of quantum chemistry to large systems: new techniques of self-consistent field theory. Full text in Research Archive
• Helgaker, Trygve (2008). The application of quantum chemistry to large systems: new techniques of self-consistent field theory. Full text in Research Archive
• Helgaker, Trygve (2008). Centre for Theoretical and Computational Chemistry. Full text in Research Archive
• Helgaker, Trygve (2008). New Techniques of Self-Consistent Field Theory. Full text in Research Archive
• Helgaker, Trygve (2008). The Dalton Quantum-Chemistry Program: Current Capabilities and Ongoing Developments. Full text in Research Archive
• Helgaker, Trygve (2008). Molecules in Strong Magnetic Fields. Full text in Research Archive
• Helgaker, Trygve (2008). Organic Quantum Chemistry. Full text in Research Archive
• Helgaker, Trygve (2008). Molecules in strong magnetic fields. Full text in Research Archive
• Helgaker, Trygve (2008). The development of methods for the calculation of the response of molecular systems to internal and external perturbations. Full text in Research Archive
• Helgaker, Trygve (2008). Methods for the calculation of internal and external perturbations of molecular systems. Full text in Research Archive
• Helgaker, Trygve (2008). Density-functional theory in quantum chemistry. Full text in Research Archive
• Helgaker, Trygve (2008). The Exhaustion of the Schrodinger equation: The accurate calculation of Molecular Properties of Light Systems. Full text in Research Archive
• Helgaker, Trygve (2008). Atomic Orbitals (1 hour), Explicit Correlation (1 hour), Basis sets (1 hour), Molecular Integrals (2 hours), DFT (4 hours), Molecular Hamiltonian (2 hours), Magnetic Properties (2 hours), Time-Independent Properties (2 hours), Benchmarking (1 hour). Full text in Research Archive
• Helgaker, Trygve (2008). Excitation energies from density-functional theory: some failures and successes. Full text in Research Archive
• Helgaker, Trygve (2007). Explicit electron correlation by a combined use of Gaussian-type orbitals and Gaussian-type geminals. Full text in Research Archive
• Helgaker, Trygve (2007). Quantum chemistry - a quantitative tool in chemistry. Full text in Research Archive
• Helgaker, Trygve (2007). Accurate Calculations and Calibration. Full text in Research Archive
• Helgaker, Trygve (2007). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2007). Time-Independent Molecular Properites. Full text in Research Archive
• Helgaker, Trygve (2007). Geomtry Optimization. Full text in Research Archive
• Helgaker, Trygve (2007). Expilcit correlation by a combined use of Gaussian-type orbitals and Gaussian-type geminals. Full text in Research Archive
• Helgaker, Trygve (2007). The Calculation of Mocular Properties of Large Systems. Full text in Research Archive
• Helgaker, Trygve (2007). The calculation of energies and properties of large systems. Full text in Research Archive
• Helgaker, Trygve (2007). Det virtuelle kjemilaboratoriet. Full text in Research Archive
• Helgaker, Trygve; Helgaker, Trygve; Dahle, Pål; Jonsson, Dan & Taylor, Peter R. (2007). Quantum-Chemical Calculations Using Gaussian-Type Orbital and Gaussian-Type Geminal Basis Sets. Full text in Research Archive
• Helgaker, Trygve (2007). The Right Answer for the Right Reason: A Tour of High-Accuracy Quantum-Chemical Calculations. Full text in Research Archive
• Helgaker, Trygve (2007). Kvantekjemi - kjemiens nye verktøy. Full text in Research Archive
• Helgaker, Trygve (2007). Strategies for large-scale molecular self-consistent field calculations. Full text in Research Archive
• Helgaker, Trygve (2007). New techniques of self-consistent field theory. Full text in Research Archive
• Helgaker, Trygve (2007). Highly accurate quantum-chemical calculations. Full text in Research Archive
• Helgaker, Trygve; Helgaker, Trygve; Dahle, Pål; Jonsson, Dan & Taylor, Peter R. (2007). Quantum-chemical calculations with Gaussian-type orbital and geminal basis sets. Full text in Research Archive
• Helgaker, Trygve (2007). New techniques of self-consistent field theory. Full text in Research Archive
• Helgaker, Trygve (2007). Relativistic Corrections in Highly Accurate Calculations on Small Molecular Systems. Full text in Research Archive
• Helgaker, Trygve (2007). Centre for Theoretical and Computational Chemistry. Full text in Research Archive
• Helgaker, Trygve (2007). Quantative Quantum Chemistry. Full text in Research Archive
• Helgaker, Trygve (2006). Self-consistent field methods for large molecular systems. Full text in Research Archive
• Reine, Simen Sommerfelt & Helgaker, Trygve (2006). Linear Scaling Density-Fitted Coulomb Evaluation. Full text in Research Archive
• Helgaker, Trygve (2006). The right answer for the wrong reason. Full text in Research Archive
• Helgaker, Trygve (2006). Hartree-Fock and Kohn-Sham Theories for Large Molecular Systems. Full text in Research Archive
• Helgaker, Trygve (2006). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2006). The Rigorous Calculation of Molecular Properties to Chemical Accuracy. Full text in Research Archive
• Helgaker, Trygve (2006). The Rigorous Calculation of Molecular Properties to Chemical Accuracy. Full text in Research Archive
• Helgaker, Trygve (2006). SCF methods for energies and properties of large molecular systems. Full text in Research Archive
• Tellgren, Erik & Helgaker, Trygve (2006). Introducing periodic boundary conditions into a molecular code. Full text in Research Archive
• Helgaker, Trygve (2006). Density-Functional Theory (2h), Basis Sets (3h), Integral Evaluation (2h), Time-Indepent Molecular Properties (3h), Magnetic Properties (4h). Full text in Research Archive
• Helgaker, Trygve (2006). Evaluation of Fock/Kohn-Sham matrices using linear-scaling techniques. Full text in Research Archive
• Helgaker, Trygve (2006). Methods for Hartree-Fock and Kohn-Sham calculations on large molecular systems. Full text in Research Archive
• Lutnæs, Ola Berg; Teale, Andrew M.; Helgaker, Trygve & Tozer, David J. (2006). Calculation of rotational g tensors using density functional theory methods. Full text in Research Archive
• Tellgren, Erik & Helgaker, Trygve (2006). Introducing periodic boundary conditions into a molecular code. Full text in Research Archive
• Rizzo, Antonio; Cappelli, Chiara; Junquera-Hernández, José Miguel; Merás, Alfredo M. J. Sánchez de; Sánchez-Marín, José & Helgaker, Trygve [Show all 7 contributors for this article] (2006). ELectric and Magnetic Properties of Boron Trihalides. Full text in Research Archive
• Reine, Simen Sommerfelt & Helgaker, Trygve (2006). Linear Scaling Density-Fitted Coulomb Evaluation. Full text in Research Archive
• Helgaker, Trygve (2005). The Accurate Calculation of Molecular Properties with Relativistic Corrections. Full text in Research Archive
• Helgaker, Trygve (2005). Molecular Magnetic Properties. Full text in Research Archive
• Helgaker, Trygve (2005). Energy derivatives and molecular properties. Full text in Research Archive
• Helgaker, Trygve (2005). Computers in chemistry education. Full text in Research Archive
• Helgaker, Trygve (2005). Geometry optimizations. Full text in Research Archive
• Helgaker, Trygve (2005). Density Functional Theory in Quantum Chemistry. Full text in Research Archive
• Helgaker, Trygve (2005). The exhaustion of the Schrödinger equation: the accurate calculation of molecular properties for light systems. Full text in Research Archive
• Helgaker, Trygve (2005). The exhaustion of the Schrödinger equation: The accurate calculation of molecular properties for light systems. Full text in Research Archive
• Coriani, Sonia; Marchesan, Domenico; Gauss, Jurgen; Hattig, Christof; Helgaker, Trygve & Jørgensen, Poul (2005). Accurate geometries from ab initio calculations: Systems containing second-row atoms and transition metals. Full text in Research Archive
• Hauge, Eirill Strand; Pedersen, Thomas Bondo & Helgaker, Trygve (2021). Extrapolating the Electric Dipole Moment. Universitetet i Oslo. Full text in Research Archive
• Jestilä, Joakim Samuel; Uggerud, Einar & Helgaker, Trygve (2021). Reductive gas-phase ion chemistry of simple oxocarbons in the presence of alkali and alkaline earth metals. Universitetet i Oslo. Full text in Research Archive Show summary

  In a circular carbon economy, carbon dioxide (CO2) is recycled and used as a molecular building block to make fuels, chemicals and materials, driven by renewable energy sources. Hence, the molecule represents a potential replacement for oil, natural gas and coal as one of the primary sources of carbon. A major bottleneck stems from the fact that CO2 requires activation before it can be converted. This work explores the activation and reduction of CO2 by the alkali metal and alkaline earth metal chloride anions on a fundamental level. It sheds light on relevant factors in the conversion of CO2 to valuable products such as oxalate (C2O42−) and carbon monoxide (CO), as well as further conversion of the latter. The aforementioned processes proceed via metal-CO2 intermediates—chemical species formed during conversion of reactants to products—and this thesis links their properties to the extent of CO2 activation. Specifically, more complete electron transfer from the metal to CO2 and larger structural changes in the intermediates are associated with lower energetic demands for further reaction and conversion. These results could prove useful for the development of efficient CO2-recycling processes.

• Hansen, Audun Skau; Pedersen, Thomas Bondo; Helgaker, Trygve & Kvaal, Simen (2021). Local correlation methods for infinite systems. Universitetet i Oslo. Full text in Research Archive Show summary

  Quantum Chemistry constitutes an extensive framework for accurately simulating the electronic wavefunction of atoms and molecules. The same framework may in principle be applied to the domain of periodic structures such as crystals, but is in practice severely limited by the infinite nature of these structures in conjunction with the computational complexity of quantum chemical methods. In his thesis, the candidate utilizes a mathematical structure known as bi-infinite block-Toeplitz matrices in order to smoothly transition between the molecular and periodic realm. Furthermore, he extends the divide-expand-consolidate methods originally devised for molecules to the periodic case, and demonstrates that this procedure can reduce the computational scaling of the simulation while retaining systematic control over the error.

• Skjelstad, Bastian; Balcells, David & Helgaker, Trygve (2020). A Computational Perspective on the Interplay Between Metal Dopant and Support in the Structure and Reactivity of Oxidized Cubanes. Universitetet i Oslo. Full text in Research Archive
• Austad, Jon; Helgaker, Trygve & Tellgren, Erik (2020). Theoretical Investigations of Molecular Electronic Structure in a Magnetic Field. Universitetet i Oslo. Full text in Research Archive Show summary

  Most available literature on molecular systems in magnetic fields are limited to special cases: Either small or highly symmetric species of specific orientation to the field, or fields of low intensity - or combinations of the above. This dissertation is devoted to the development, implementation, calibration, analysis and usage of computational quantum chemistry methods that are freed from such restrictions. Three research papers are presented. One deals with the inner workings of a generalized variant of Density Functional Theory which allows the magnetic field to be directly included (BDFT), and explores in depth how various functionals handle diamagnetic, paramagnetic and aromatic molecules. The second paper investigates the rich chemistry of the helium dimer in a strong magnetic field, thoroughly maps out the electronic structure of the molecule, and presents the many different magnetic bonding mechanisms and interactions that exist in this regime. The final paper revolves around application of highly accurate wave-function methods to determine the influence of terrestrially available magnetic fields on water. The work of this thesis presents important contributions to theoretical chemistry in the pres- ence of magnetic fields.

• Peltzer, Raphael Mathias; Cascella, Michele; Pedersen, Thomas Bondo & Helgaker, Trygve (2018). Multi-scale computational modelling of bio-chemical systems in the condensed phase. Universitetet i Oslo. Full text in Research Archive
• Miller, Glenn B. S.; Uggerud, Einar & Helgaker, Trygve (2018). Intrinsic Reactivity of Reduced Mg and Zn with CO2 – Relationships to Biological and Non-Biological C–C Bond Formation. Universitetet i Oslo. Full text in Research Archive
• Bunkan, Arne Joakim Coldevin; Nielsen, Claus Jørgen; Uggerud, Einar & Helgaker, Trygve (2015). A theoretical and experimental study of atmospheric reactions of amines and their degradation products. Universitetet i Oslo. Full text in Research Archive Show summary

  Amine-based CO2 capture has been proposed as a way of mitigating climate change. In this work, the fundamental processes governing the atmospheric gas phase chemistry of amines released from CO2 capture have been studiedusing computational and experimental methods. The reactions of amines, amides and imines with OH radicals have been studied with computational methods. The reactions are all characterized by strongly bound pre-reaction complexes and submerged barriers to hydrogen abstraction. Rate coefficients calculated from statistical rate theory show negative temperature dependencies and are in good agreement with experimental results where available. It has been suggested that the climate effect of amine based CO2 capture may be counteracted by the formation of N2O as a minor product in the photo-oxidation of methylamine. Calculations in this work unambiguously show that N2O is not a product in the photo-oxidation of methylamine. The reaction rate coefficients for the OH radical reactions of N-methylformamide and N,N-dimethylformamide have been measured as a function of temperature and pressure by using the flash photolysis laser-induced fluorescence technique. The reactions are significantly slower than the corresponding amine reactions and show a negative temperature dependence. The study further reveals that the amides have a higher potential for nitramine and nitrosamine formation and that they may therefore constitute a previously undisclosed health risk. Kinetic isotope effects for the OH radical reaction of HCN have been measured in a smog chamber through the use of FTIR spectroscopy. The measured values and calculated results confirm that the reaction, unlike the other reactions in this work, mainly occur through an addition channel. The present work has contributed significantly to our understanding of the atmospheric reactions of the intermediates in the atmospheric degradation of methyl amines.

• Trøyflat, Jostein; Helgaker, Trygve; Tellgren, Erik & Reine, Simen Sommerfelt (2015). Efficient methods for self-consistent field calculation of molecules in finite magnetic fields. Universitetet i Oslo. Full text in Research Archive
• Rybkin, Vladimir; Helgaker, Trygve & Uggerud, Einar (2014). Development of Direct Born–Oppenheimer Molecular Dynamics with Applications to Water Clusters and Mechanochemistry. Universitetet i Oslo. Full text in Research Archive Show summary

  New methods for Born-Oppenheimer molecular dynamics simulations have been developed and used to study protonated water clusters and mechanochemistry of alkanes. The novel algorithms are an efficient coordinate transformation and sampling microcanonical ensembles of trajectories used for generating starting conditions for simulations. In combination with existing techniques as well as with electronic structure methods within DALTON program, they make it a powerful tool to study reaction dynamics, free of charge. This has been demonstrated by investigation of protonated water clusters relevant for water nucleation in atmospheric chemistry. The structure of the proton salvation shell and evaporation kinetics have been revealed. Another application is the dynamics of alkanes under mechanical stress. Short-chain alkanes – butane and octane – are model compounds for polymers. Their dynamical behavior was simulated and analyzed under two scenarios mimicking the modes ofmechanochemical activation: slow stretching in the atomic-force microscope and violent action of ultrasound.

• Zatula, Alexey; Uggerud, Einar; Helgaker, Trygve & Samdal, Svein (2012). Experimental and computational studies of dynamic processes in ionic water clusters. Universitetet i Oslo. Full text in Research Archive Show summary

  Clusters of water molecules mimic the transition from gas phase to bulk water. Cluster species of the desired sizes can be selected using mass spectrometric techniques and their size-dependent properties can thereby be measured. The properties of small ionic clusters are of particular relevance to atmospheric science, providing insights into nucleation phenomena. This work is dedicated to the investigation of properties of selected ionic molecular clusters and their gas phase reactions with heavy water or ammonia, with a strong emphasis on proton transfer phenomena. This has been achieved both experimentally in cluster beam experiments and by quantum chemical calculations. Both in the abundance spectra and the evaporation patterns of the investigated aqueous clusters “magic numbers” discontinuities in otherwise smoothly varying distributions were observed, and are discussed. To further examine a marked difference in the observed “magic-number” behaviour of H+(pyridine)1(H2O)n and H+(NH3)1(pyridine)1(H2O)n clusters, quantum chemical calculations have been employed. Next, relative reaction cross sections were measured for cluster ions reacting with D2O and with NH3 in the collision cell. Analysis of the results for the reaction H+(pyridine)1(H2O)n + NH3 allowed us to improve a kinetic model of the atmospheric positive ion composition. Upon reaction of a cluster with D2O a short-lived reaction intermediate is formed, which is followed by subsequent loss of D2O, HDO or H2O. The reaction channel leading to the loss of HDO requires proton mobility within the cluster, involving O–H-bond activation. The loss of HDO was not observed for protonated water clusters containing one pyridine molecule, a consequence of the immobilizing effect on the extra proton by the nitrogen base site. Similarly, the rates of protium/deuterium exchange for water clusters containing alkali metal ions are consistently extremely low. However, the experiments show enhanced proton mobility in water clusters containing two or three pyridine molecules (H+(pyridine)2–3(H2O)n), in 2,2'-bipyridine and 2,2'-ethylenebipyridine containing water clusters as well as in bisulfate water clusters (HSO4?(H2O)n). On the basis of systematic quantum chemical calculations we present consistent mechanisms for low energy water rearrangement and proton transfer along preformed "wires" of hydrogen bonds between the two distinct sites provided by these core ions in complete support of the experimental findings.

• Larsen, Kjersti Beate; Fjellvåg, Helmer; Bakken, Vebjørn; Ponniah, Ravindran & Helgaker, Trygve (2011). DFT-modellering av elektroniske og optiske egenskaper i CPO-27-Zn. Universitetet i Oslo. Full text in Research Archive Show summary

  The aim of this project was to achieve more knowledge and a better understanding of the physical and the chemical properties of the one-dimensional chains in zinc oxide as a way to apporoximate nanowires of zinc oxide. The main focus has been to understand the electronical and optical properties, which has been studied by theoretical as well as experimental methods. The compound used as a starting point to study these properties has chemical formula [Zn2(dhtp)(H2O)2] . 8 H2O and is denoted in the thesis as Coordination Polymer of Oslo number 27 type Zn (CPO-27-Zn). The zinc atom is six-coordinated at room temperature, resulting in ZnO6-units where five of the oxygen atoms stems from the organic ligands, and the remaining sixth is part of a water molecule. Upon dehydration, the structure of the compound changes substantially, which reduces the coordination number of the zinc atom to five from six. Cluster models using the experimental coordinates of CPO-27-Zn were constructed, however, it was difficult to make representative models, and the results obtained from the different methods showed large deviations. As part of the theoretical work, both non-periodic and periodic calculations was performed using the software packages Gaussian09 and VASP. The results from the non-periodic calculations indicates a larger HOMO-LUMO gap in the dehydrated models of CPO-27-Zn compared to the hydrated ones, however, the estimated bandgaps from the periodic calculations show the opposite. As a result of this, ultraviolet visual diffuse reflectance spectroscopy measurements were performed on CPO-27-M (M=Zn, Co, Ni, Mg, Mn) to achieve more knowledge of how the bandgap is affected in this material. The conclusion from the experimental study is that the bandgap remains apporoximately unchanged in CPO-27-Zn and CPO-27-Mg, while there is a significant drop in the bandgap in CPO-27-Mn. However, the change of bandgap upon dehydration of CPO-27-Zn, which also causes a change in the coordination number of the zinc atom, is very small. This is in good agreement with previous published results, but does not agree with any of the conclusions from the calculations. Therefore, the models constructed does not fully represent the compound CPO-27-Zn.

• Gisler, Galen Ross; Celledoni, Elena; Helgaker, Trygve; Iversen, Trond; Jakobsen, Kjetill Sigurd & Jones, Colin N. [Show all 12 contributors for this article] (2010). The scientific case for eInfrastructure in Norway. Norges forskningsråd. ISSN 9788212028319. Full text in Research Archive Show summary

  Fulltekst: http://www.forskningsradet.no/servlet/Satellite?blobcol=urldata&blobheader=application%2Fpdf&blobheadername1=Content-Disposition%3A&blobheadervalue1=+attachment%3B+filename%3DeSOP-The-Scientific-Case-for-eInfrastructure-in-Norway-final%5B1%5D.pdf&blobkey=id&blobtable=MungoBlobs&blobwhere=1274464831935&ssbinary=true

• Lutnæs, Ola Berg; Helgaker, Trygve & Tozer, David J. (2009). Density-functional-theory calculations of second-order magnetic properties. Universitetet i Oslo. Full text in Research Archive Show summary

  To concluse, the work presented in this thesis is a contribution to the understanding and testing of DFT functionals (and also other computational methods) for the calculation of second-order magnetic properties. Vibrational contributions to the properties,in addition to be interesting in their own right, are highly important in the benchmark process in order to bridge quantum-chemical calculations to experimental measurements. Several DFT functionals and basis sets have been tested for magnetic property calculations and give directions on the choice of functional and basis set for practical application work. For the rotational g-tensor and magnetizability, a benchmark set of high quality coupled-cluster calculations has been developed and is available for future benchmark studies. The proper benchmarking of DFT methods gives directions to both new functional developments and the choice of functional in applications.

• Reine, Simen Sommerfelt; Helgaker, Trygve & Pawlowski, Filip (2009). Density functional theory for large molecular systems. Universitetet i Oslo. Full text in Research Archive Show summary

  Accurate simulations of chemical and biological processes at a molecular level have long been missing for a wide range of molecular systems, but in now possible for many of these systems due to the recent developments of Simen Reine, Trygve Helgaker and co-workers at the University of Oslo. Computer simulations are widely used in chemistry and related fields like biology, pharmacy and medicine. Quantum-chemical methods are the basis of the most accurate simulation techniques. They are of great help in the determination and prediction of molecular properties e.g. molecular structures and give important and detailed insights into chemical reactions - both qualitatively and quantitatively. The predictive power and the range of molecular systems these methods can be applied to is closely related to their accuracy, efficiency and user-friendliness, and therefore the development of new and improved methods increases the impact of quantum chemistry to an even greater extent. Such developments enable treatment of systems previously out of reach, improve the description of the systems accessible by standard methodology, and as a consequence, reduce the need for time demanding and costly experiments. Most quantum chemical calculations are carried out using density functional theory (DFT), as this method constitutes a good compromise between accuracy and performance. Although highly useful, current methodology was limited to molecules consisting of up to a few hundred atoms, and the description of for example proteins has therefore been restricted to a rather crude level. In the thesis "Density functional theory for large molecular systems" new methods have been developed aiming at routine DFT calculations of large systems. Molecular calculations on systems containing 1400 atoms are here reported - and the methods have since been applied to systems containing up to 4000 atoms.

• Tellgren, Erik; Helgaker, Trygve; Ruud, Kenneth & Soncini, Alessandro (2009). Quantum-chemical method development for extended and magnetic systems. Universitetet i Oslo. Full text in Research Archive
• Gruber-Stadler, Margret; Nielsen, Claus Jørgen; Helgaker, Trygve & Mühlhäuser, Max (2008). Quantum chemistry studies of elementary atmospheric key reactions. Universitetet i Oslo. Full text in Research Archive
• Strenalyuk, Tatyana; Samdal, Svein & Helgaker, Trygve (2008). Molecular structure of phthalocyanine complexes using gas-phase electron diffraction and quantum chemical calculations. Universitetet i Oslo. Full text in Research Archive

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