Thomas Bondo Pedersen

• Adamowicz, Ludwik; Kvaal, Simen; Lasser, Caroline & Pedersen, Thomas Bondo (2022). Laser-induced dynamic alignment of the HD molecule without the Born-Oppenheimer approximation. Journal of Chemical Physics. ISSN 0021-9606. doi: 10.1063/5.0101352. Full text in Research Archive Show summary

  Laser-induced molecular alignment is well understood within the framework of the Born-Oppenheimer (BO) approximation. Without the BO approximation, however, the concept of molecular structure is lost, making alignment hard to define precisely. In this work, we demonstrate the emergence of alignment from the first-ever non-BO quantum dynamics simulations, using the HD molecule exposed to ultrashort laser pulses as a few-body test case. We extract the degree of alignment from the non-BO wave function by means of an operator expressed in terms of pseudo-proton coordinates that mimics the BO-based definition of alignment. The only essential approximation, in addition to the semiclassical electric-dipole approximation for the matter-field interaction, is the choice of time-independent explicitly correlated Gaussian basis functions. We use a variational, electric-field-dependent basis-set construction procedure, which allows us to keep the basis-set dimension low whilst capturing the main effects of electric polarization on the nuclear and electronic degrees of freedom. The basis-set construction procedure is validated by comparing with virtually exact grid-based simulations for two one-dimensional model systems: laser-driven electron dynamics in a soft attractive Coulomb potential and nuclear rovibrational dynamics in a Morse potential.

• Kristiansen, Håkon Emil; Ofstad, Benedicte; Hauge, Eirill Strand; Aurbakken, Einar; Schøyen, Øyvind Sigmundson & Kvaal, Simen [Show all 7 contributors for this article] (2022). Linear and Nonlinear Optical Properties from TDOMP2 Theory. Journal of Chemical Theory and Computation. ISSN 1549-9618. 18(6), p. 3687–3702. doi: 10.1021/acs.jctc.1c01309. Full text in Research Archive
• Hansen, Audun Skau; Aurbakken, Einar & Pedersen, Thomas Bondo (2021). Smooth potential-energy surfaces in fragmentation-based local correlation methods for periodic systems. Molecular Physics. ISSN 0026-8976. 119(9). doi: 10.1080/00268976.2021.1896046. Full text in Research Archive Show summary

  Local approximations facilitate the application of post-Hartree–Fock methods in the condensed phase, but simultaneously introduce errors leading to discontinuous potential-energy surfaces. In this work, we explore how these discontinuities arise in periodic systems, their implications, and possible ways of controlling them. In addition, we present a fully periodic Divide-Expand-Consolidate second-order Møller–Plesset approach using an attenuated resolution-of-the-identity approximation for the electron repulsion integrals and a convenient class to handle translation-symmetric tensors in block-Toeplitz format.

• Pedersen, Thomas Bondo; Kristiansen, Håkon Emil; Bodenstein, Tilmann; Kvaal, Simen & Schøyen, Øyvind Sigmundson (2021). Interpretation of Coupled-Cluster Many-Electron Dynamics in Terms of Stationary States. Journal of Chemical Theory and Computation. ISSN 1549-9618. 17(1), p. 388–404. doi: 10.1021/acs.jctc.0c00977. Full text in Research Archive Show summary

  We demonstrate theoretically and numerically that laser-driven many-electron dynamics, as described by bivariational time-dependent coupled-cluster theory, may be analyzed in terms of stationary-state populations. Projectors heuristically defined from linear response theory and equation-of-motion coupled cluster theory are proposed for the calculation of stationary-state populations during interaction with laser pulses or other external forces, and conservation laws of the populations are discussed. Numerical tests of the proposed projectors, involving both linear and nonlinear optical processes for the He and Be atoms, and for the LiH, CH+, and LiF molecules, show that the laser-driven evolution of the stationary-state populations at the coupled-cluster singles-and-doubles (CCSD) level is very close to that obtained by ful configuration-interaction theory provided all stationary states actively participating in the dynamics are sufficiently well approximated. When double-excited states are important for the dynamics, the quality of the CCSD results deteriorate. Observing that populations computed from the linear-response projector may show spurious small-amplitude, high-frequency oscillations, the equation-of-motion projector emerges as the most promising approach to stationary-state populations.

• Aquilante, Francesco; Autschbach, Jochen; Baiardi, Alberto; Battaglia, Stefano; Borin, Veniamin A. & Chibotaru, Liviu F. [Show all 39 contributors for this article] (2020). Modern quantum chemistry with [Open]Molcas. Journal of Chemical Physics. ISSN 0021-9606. 152(21). doi: 10.1063/5.0004835. Full text in Research Archive
• Olsen, Jógvan Magnus Haugaard; Reine, Simen Sommerfelt; Vahtras, Olav; Kjellgren, Erik; Reinholdt, Peter & Dundas, Karen O.H [Show all 24 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
• Hansen, Audun Skau; Baardsen, Gustav; Rebolini, Elisa; Maschio, Lorenzo & Pedersen, Thomas Bondo (2020). Representation of the virtual space in extended systems – a correlation energy convergence study. Molecular Physics. ISSN 0026-8976. doi: 10.1080/00268976.2020.1733118. Full text in Research Archive
• Kristiansen, Håkon Emil; Schøyen, Øyvind Sigmundson; Kvaal, Simen & Pedersen, Thomas Bondo (2020). Numerical stability of time-dependent coupled-cluster methods for many-electron dynamics in intense laser pulses. Journal of Chemical Physics. ISSN 0021-9606. 152. doi: 10.1063/1.5142276. Full text in Research Archive
• Pedersen, Thomas Bondo & Kvaal, Simen (2019). Symplectic integration and physical interpretation of time-dependent coupled-cluster theory. Journal of Chemical Physics. ISSN 0021-9606. 150. doi: 10.1063/1.5085390. Full text in Research Archive Show summary

  The formulation of the time-dependent Schrödinger equation in terms of coupled-cluster theory is outlined, with emphasis on the bivariational framework and its classical Hamiltonian structure. An indefinite inner product is introduced, inducing physical interpretation of coupled-cluster states in the form of transition probabilities, autocorrelation functions, and explicitly real values for observables, solving interpretation issues which are present in time-dependent coupled-cluster theory and in ground-state calculations of molecular systems under the influence of external magnetic fields. The problem of the numerical integration of the equations of motion is considered, and a critical evaluation of the standard fourth-order Runge–Kutta scheme and the symplectic Gauss integrator of variable order are given, including several illustrative numerical experiments. While the Gauss integrator is stable even for laser pulses well above the perturbation limit, our experiments indicate that a system-dependent upper limit exists for the external field strengths. Above this limit, time-dependent coupled-cluster calculations become very challenging numerically, even in the full configuration interaction limit. The source of these numerical instabilities is shown to be rapid increases of the amplitudes as ultrashort high-intensity laser pulses pump the system out of the ground state into states that are virtually orthogonal to the static Hartree-Fock reference determinant.

• Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2018). Divide-Expand-Consolidate Second-Order Møller-Plesset Theory with Periodic Boundary Conditions. Journal of Chemical Theory and Computation. ISSN 1549-9618. 14(5), p. 2427–2438. doi: 10.1021/acs.jctc.8b00021. Full text in Research Archive
• Aquilante, Francesco; Delcey, Mickaël G.; Pedersen, Thomas Bondo; Fdez Galvan, Ignacio & Lindh, Roland (2017). Inner projection techniques for the low-cost handling of two-electron integrals in quantum chemistry. Molecular Physics. ISSN 0026-8976. 115(17-18), p. 2052–2064. doi: 10.1080/00268976.2017.1284354. Full text in Research Archive
• Sinha, Shashi Bhushan; Shopov, Dimitar Y; Sharninghausen, Liam S.; Stein, Christopher J.; Mercado, Brandon Q. & Balcells, David [Show all 10 contributors for this article] (2017). Redox Activity of Oxo-Bridged Iridium Dimers in an N,O-Donor Environment: Characterization of Remarkably Stable Ir(IV,V) Complexes. Journal of the American Chemical Society. ISSN 0002-7863. 139(28), p. 9672–9683. doi: 10.1021/jacs.7b04874.
• Wirz, Lukas Nico; Reine, Simen Sommerfelt & Pedersen, Thomas Bondo (2017). On Resolution-of-the-Identity Electron Repulsion Integral Approximations and Variational Stability. Journal of Chemical Theory and Computation. ISSN 1549-9618. 13(10), p. 4897–4906. doi: 10.1021/acs.jctc.7b00801. Full text in Research Archive
• Galván, Ignacio Fdez.; Delcey, Mickaël G.; Pedersen, Thomas Bondo; Aquilante, Francesco & Lindh, Roland (2016). Analytical State-Average Complete-Active-Space Self-Consistent Field Nonadiabatic Coupling Vectors: Implementation with Density-Fitted Two-Electron Integrals and Application to Conical Intersections. Journal of Chemical Theory and Computation. ISSN 1549-9618. 12(8), p. 3636–3653. doi: 10.1021/acs.jctc.6b00384.
• 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.
• Aquilante, Francesco; Autschbach, Jochen; Carlson, Rebecca K.; Chibotaru, Liviu F.; Delcey, Mickaël G. & de Vico, Luca [Show all 38 contributors for this article] (2016). Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table. Journal of Computational Chemistry. ISSN 0192-8651. 37(5), p. 506–541. doi: 10.1002/jcc.24221.
• Freitag, Leon; Knecht, Stefan; Keller, Sebastian F.; Delcey, Mickaël G.; Aquilante, Francesco & Pedersen, Thomas Bondo [Show all 9 contributors for this article] (2015). Orbital entanglement and CASSCF analysis of the Ru-NO bond in a Ruthenium nitrosyl complex. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 17(22), p. 14383–14392. doi: 10.1039/c4cp05278a. Full text in Research Archive
• Delcey, Mickaël G.; Pedersen, Thomas Bondo; Aquilante, Francesco & Lindh, Roland (2015). Analytical gradients of the state-average complete active space self-consistent field method with density fitting. Journal of Chemical Physics. ISSN 0021-9606. 143(4). doi: 10.1063/1.4927228.
• Freitag, Leon; Knecht, Stefan; Keller, Sebastian F.; Delcey, Mickaël G.; Aquilante, Francesco & Pedersen, Thomas Bondo [Show all 9 contributors for this article] (2015). Erratum: Orbital entanglement and CASSCF analysis of the Ru-NO bond in a Ruthenium nitrosyl complex(Phys. Chem. Chem. Phys. (2015 ) DOI: 10.1039/c4cp05278a). Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 17(20). doi: 10.1039/c5cp90073e.
• Delcey, Mickaël G.; Aquilante, Francesco; Pedersen, Thomas Bondo & Lindh, Roland (2014). Analytical CD/RI-SA-CASSCF gradients: Implementation and performance. Abstract of Papers of the American Chemical Society. ISSN 0065-7727. 248.
• Delcey, Mickaël G.; Freitag, Leon; Pedersen, Thomas Bondo; Aquilante, Francesco; Lindh, Roland & González, Leticia (2014). Analytical gradients of complete active space self-consistent field energies using Cholesky decomposition: Geometry optimization and spin-state energetics of a ruthenium nitrosyl complex. Journal of Chemical Physics. ISSN 0021-9606. 140(17). doi: 10.1063/1.4873349. Show summary

  We present a formulation of analytical energy gradients at the complete active space self-consistent field (CASSCF) level of theory employing density fitting (DF) techniques to enable efficient geometry optimizations of large systems. As an example, the ground and lowest triplet state geometries of a ruthenium nitrosyl complex are computed at the DF-CASSCF level of theory and compared with structures obtained from density functional theory (DFT) using the B3LYP, BP86, and M06L functionals. The average deviation of all bond lengths compared to the crystal structure is 0.042 Å at the DF-CASSCF level of theory, which is slightly larger but still comparable with the deviations obtained by the tested DFT functionals, e.g., 0.032 Å with M06L. Specifically, the root-mean-square deviation between the DF-CASSCF and best DFT coordinates, delivered by BP86, is only 0.08 Å for S0 and 0.11 Å for T1, indicating that the geometries are very similar. While keeping the mean energy gradient errors below 0.25%, the DF technique results in a 13-fold speedup compared to the conventional CASSCF geometry optimization algorithm. Additionally, we assess the singlet-triplet energy vertical and adiabatic differences with multiconfigurational second-order perturbation theory (CASPT2) using the DF-CASSCF and DFT optimized geometries. It is found that the vertical CASPT2 energies are relatively similar regardless of the geometry employed whereas the adiabatic singlet-triplet gaps are more sensitive to the chosen triplet geometry.

• Boström, Jonas; Veryazov, Valera; Aquilante, Francesco; Pedersen, Thomas Bondo & Lindh, Roland (2014). Analytical gradients of the second-order Møller-Plesset energy using Cholesky decompositions. International Journal of Quantum Chemistry. ISSN 0020-7608. 114(5), p. 321–327. doi: 10.1002/qua.24563.
• Aidas, Kestutis; Angeli, Celestino; Bak, Keld L.; Bakken, Vebjørn; Bast, Radovan & Boman, Linus [Show all 82 contributors for this article] (2014). The Dalton quantum chemistry program system. Wiley Interdisciplinary Reviews. Computational Molecular Science. ISSN 1759-0876. 4(3), p. 269–284. 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.

• 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.
• 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. 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.

• Boström, Jonas; Aquilante, Francesco; Pedersen, Thomas Bondo & Lindh, Roland (2013). Analytical Gradients of Hartree-Fock Exchange with Density Fitting Approximations. Journal of Chemical Theory and Computation. ISSN 1549-9618. 9(1), p. 204–212. doi: 10.1021/ct200836x.
• Boström, Jonas; Pitonak, Michal; Aquilante, Francesco; Neogrady, Pavel; Pedersen, Thomas Bondo & Lindh, Roland (2012). Coupled Cluster and Moller-Plesset Perturbation Theory Calculations of Noncovalent Intermolecular Interactions using Density Fitting with Auxiliary Basis Sets from Cholesky Decompositions. Journal of Chemical Theory and Computation. ISSN 1549-9618. 8(6), p. 1921–1928. doi: 10.1021/ct30030181.
• Pedersen, Thomas Bondo (2012). Introduction to Response Theory. In Leszczynski, Jerzy (Eds.), Handbook of Computational Chemistry. Springer Science+Business Media B.V.. ISSN 978-94-007-0711-5. p. 135–156.
• 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.
• Cacheiro, Javier Lopez; Pedersen, Thomas Bondo; Fernández, Berta; Sánchez de Merás, Alfredo & Koch, Henrik (2011). The CCSD(T) model with Cholesky decomposition of orbital energy denominators. International Journal of Quantum Chemistry. ISSN 0020-7608. 111(2), p. 349–355. doi: 10.1002/qua.22582.
• Aquilante, Francesco; Boman, Linus; Boström, Jonas; Koch, Henrik; Lindh, Roland & Sánchez de Merás, Alfredo [Show all 7 contributors for this article] (2011). Cholesky decomposition techniques in electronic structure theory. In Zalesny, Robert; Papadopoulos, Manthos; Mezey, Paul G. & Leszczynski, Jerzy (Ed.), Linear-Scaling Techniques in Computational Chemistry and Physics. Springer Science+Business Media B.V.. ISSN 978-90-481-2853-2. p. 301–343. doi: 10.1007/978-90-481-2853-2_13.
• Aquilante, Francesco; de Vico, Luca; Ferre, Nicolas; Ghigo, Giovanni; Malmqvist, Per-Åke & Neogrady, Pavel [Show all 14 contributors for this article] (2010). MOLCAS 7: The Next Generation. Journal of Computational Chemistry. ISSN 0192-8651. 31(1), p. 224–247. doi: 10.1002/jcc.21318.
• Boström, Jonas; Delcey, Mickaël G.; Aquilante, Francesco; Serrano-Andres, Luis; Pedersen, Thomas Bondo & Lindh, Roland (2010). Calibration of Cholesky Auxiliary Basis Sets for Multiconfigurational Perturbation Theory Calculations of Excitation Energies. Journal of Chemical Theory and Computation. ISSN 1549-9618. 6(3), p. 747–754. doi: 10.1021/ct900612k.
• Pedersen, Thomas Bondo; Kongsted, Jacob & Crawford, T. Daniel (2009). Gas phase optical rotation calculated from coupled cluster theory with zero-point vibrational corrections from density funcional theory. Chirality. ISSN 0899-0042. 21, p. S68–S75. doi: 10.1002/chir.20778.
• Aquilante, Francesco; Todorova, Tanya Kumanova; Gagliardi, Laura; Pedersen, Thomas Bondo & Roos, Björn O (2009). Systematic truncation of the virtual space in multiconfigurational perturbation theory. Journal of Chemical Physics. ISSN 0021-9606. 131. doi: 10.1063/1.3157463.
• Boström, Jonas; Aquilante, Francesco; Pedersen, Thomas Bondo & Lindh, Roland (2009). Ab Initio Density Fitting: Accuracy Assessment of Auxiliary Basis Sets from Cholesky Decompositions. Journal of Chemical Theory and Computation. ISSN 1549-9618. 5(6), p. 1545–1553. doi: 10.1021/ct9000284.
• Aquilante, Francesco; Gagliardi, Laura; Pedersen, Thomas Bondo & Lindh, Roland (2009). Atomic Cholesky decompositions: A route to unbiased auxiliary basis sets for density fitting approximation with tunable accuracy and efficiency. Journal of Chemical Physics. ISSN 0021-9606. 130. doi: 10.1063/1.3116784.
• Pedersen, Thomas Bondo; Kongsted, Jacob; Crawford, T. Daniel & Ruud, Kenneth (2009). On the importance of vibrational contributions to small-angle optical rotation: Fluoro-oxirane in gas phase and solution. Journal of Chemical Physics. ISSN 0021-9606. 130. doi: 10.1063/1.3054301.

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• Kristiansen, Håkon Emil; Ofstad, Benedicte Sverdrup; Hauge, Eirill Strand; Aurbakken, Einar; Schøyen, Øyvind Sigmundson & Kvaal, Simen [Show all 7 contributors for this article] (2022). Correction to “Linear and Nonlinear Optical Properties from TDOMP2 Theory”. Journal of Chemical Theory and Computation. ISSN 1549-9618. 18(9), p. 5755–5757. doi: 10.1021/acs.jctc.2c00830.
• Pedersen, Thomas Bondo (2022). From Electronic Dynamics to Fully Coupled Electronic-Nuclear Dynamics.
• Pedersen, Thomas Bondo (2022). From Electronic Dynamics to Fully Coupled Electronic-Nuclear Dynamics.
• Hauge, Eirill Strand & Pedersen, Thomas Bondo (2021). Extrapolating the electric dipole moment of real-time electronic structure calculations.
• Pedersen, Thomas Bondo (2021). Interpretation of Coupled-Cluster Quantum Dynamics.
• Pedersen, Thomas Bondo (2020). Laser-Driven Many-Electron Dynamics with Coupled-Cluster Theory.
• Pedersen, Thomas Bondo (2019). Can Coupled-Cluster Theory Describe Ultrashort, High-Intensity Laser-Driven Electron Dynamics?
• Pedersen, Thomas Bondo (2019). Electron Dynamics in Extreme Laser Pulses: A Challenge to Coupled-Cluster Theory.
• Pedersen, Thomas Bondo & Kvaal, Simen (2018). Electron Dynamics with Coupled-Cluster Theory.
• Pedersen, Thomas Bondo & Kvaal, Simen (2018). Time-Dependent Coupled-Cluster Theory.
• Baardsen, Gustav; Rebolini, Elisa; Hansen, Audun Skau; Maschio, Lorenzo; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2018). The divide-expand-consolidate method for extended systems.
• Pedersen, Thomas Bondo (2018). New Directions in Coupled-Cluster Theory.
• Hansen, Audun Skau; Baardsen, Gustav; Maschio, Lorenzo & Pedersen, Thomas Bondo (2018). Locality and sparsity in local correlation calculations.
• Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2018). Error-controlled MP2 for periodic molecular systems.
• Pedersen, Thomas Bondo (2017). Coupled-cluster theory: Quo Vadis?
• Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2017). Local coupled cluster methods for periodic systems.
• Pedersen, Thomas Bondo (2017). Local Coupled-Cluster Theory with Periodic Boundary Conditions.
• Pedersen, Thomas Bondo (2017). Divide-Expand-Consolidate Coupled-Cluster Theory for Extended Systems.
• Pedersen, Thomas Bondo (2017). Esoteric or Exoteric? Gauge Invariance, Time Propagation, and Coupled-Cluster Theory.
• Pedersen, Thomas Bondo (2016). Local coupled-cluster methods for periodic systems.
• Leikanger, Karl R.; Rebolini, Elisa; Hansen, Audun Skau; Baardsen, Gustav & Pedersen, Thomas Bondo (2016). Hartree-Fock calculations using a priori Wannier orbitals for solids.
• 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.
• 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.
• Galván, Ignacio Fdez.; Delcey, Mickaël G.; Pedersen, Thomas Bondo; Aquilante, Francesco & Lindh, Roland (2016). Conical intersection optimization and characterization with density-fitted two-electron integrals.
• Pedersen, Thomas Bondo (2016). Coupled cluster methods for periodic systems.
• Pedersen, Thomas Bondo (2016). A quantum chemistry approach to condensed phases using periodic boundary conditions.
• Pedersen, Thomas Bondo (2015). On the pair atomic resolution of the identity approximation.
• Pedersen, Thomas Bondo (2015). On the Pair Atomic Resolution of the Identity Approximation.
• Pedersen, Thomas Bondo (2014). Accelerating Quantum Chemistry using Local Density Fitting.
• Pedersen, Thomas Bondo (2014). The Random Phase Approximation.
• Pedersen, Thomas Bondo; Rekkedal, Johannes Andreas; Leikanger, Karl R.; Iozzi, Maria Francesca; Coriani, Sonia & Teale, Andrew Michael (2014). Molecular Properties in the Random Phase Approximation.
• Pedersen, Thomas Bondo (2014). The CD project - a status report.
• Pedersen, Thomas Bondo (2013). Cholesky decomposition and density fitting in Molcas.
• Pedersen, Thomas Bondo (2013). The Random Phase Approximation.
• Pedersen, Thomas Bondo; Rekkedal, Johannes Andreas; Iozzi, Maria Francesca; Coriani, Sonia; Teale, Andrew Michael & Helgaker, Trygve (2013). Molecular Properties in the Random Phase Approximation.
• Kristiansen, Håkon Emil; Pedersen, Thomas Bondo; Kvaal, Simen & Ruud, Kenneth (2022). Coupled-Cluster Theory for Electron Dynamics. Department of Chemistry, Faculty of Mathematics and Natural Sciences, University of Oslo. ISSN 1501-7710. 2022(2507). Show summary

  Coupled-cluster (CC) theory, often referred to as the gold standard of quantum chemistry, defines a convergent hierarchy of increasingly accurate methods for the description of molecular properties. The same hierarchy of methods can be extended to time-dependent electronic structure theory which is then referred to as time-dependent CC theory (TDCC). TDCC theory can be formulated with static orbitals or with dynamic orbitals. In his thesis, the candidate has developed software implementing both variants. An assessment of the importance of employing dynamic orbitals is given. A drawback of TDCC theory is the difficulty of interpretation due to the non-Hermitian formulation. For example, there is no fully consistent definition of excited states in CC theory, preventing the calculation of stationary-state populations. Based on equation-of-motion CC (EOMCC) and CC linear response (CCLR) theory, we propose two sets of projection operators that yield time-dependent stationary-state populations as expectation values. Furthermore, TDCC methods are computationally expensive. To bring reduce the computational cost, approximations to specific orders of the theory can be formulated based on perturbation theory. The candidate has developed a program for a second-order approximation to TDCC theory with dynamic orbitals and applied the method to the description of optical properties in small molecules.

• Hauge, Eirill Strand; Pedersen, Thomas Bondo & Helgaker, Trygve (2021). Extrapolating the Electric Dipole Moment. Universitetet i Oslo, Det matematisk-naturvitenskapelige fakultet.
• Hansen, Audun Skau; Pedersen, Thomas Bondo; Helgaker, Trygve & Kvaal, Simen (2021). Local correlation methods for infinite systems. Universitetet i Oslo, Det matematisk-naturvitenskapelige fakultet. ISSN 1501-7710. 2021(2380). Full text in Research Archive
• Aurbakken, Einar; Pedersen, Thomas Bondo; Fredianini, Luca & Hjorth-Jensen, Morten (2020). Local Møller-Plesset perturbation theory for periodic systems - a study of long-range interactions and convergence. Kjemisk Institutt, Universitetet i Oslo.
• 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. ISSN 1501-7710. 2018(2058).

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