stellsto

• Grazioli, Laura; Kitsaras, Marios Petros & Stopkowicz, Stella (2025). Unitary Coupled-Cluster Theory for the Treatment of Molecules in Strong Magnetic Fields. Journal of Chemical Theory and Computation. ISSN 1549-9618. 21(24), s. 12634–12651. doi: 10.1021/acs.jctc.5c01521. Fulltekst i vitenarkiv Vis sammendrag

  In coupled-cluster (CC) theory, unphysical complex energies may arise in the presence of strong magnetic fields, near conical intersections, or in systems exhibiting complex Abelian point group symmetries. This issue originates from the non-Hermitian nature of the CC energy expression. A promising solution is provided by unitary coupled-cluster (UCC) theory, which retains the advantages of an exponential parametrization while ensuring real-valued energy eigenvalues. In this work, we present an implementation of finite-field second-order (ff-UCC2) and third-order (ff-UCC3) UCC theory. We assess the performance of these truncation levels in comparison to conventional finite-field CC methods, using the methylidyne ion, water, and boric acid.

• Farhaz, Raunak; Bischoff, Florian A.; Blaschke, Simon & Stopkowicz, Stella (2025). Quantification of the basis set error for molecules in strong magnetic fields and general orientation. Journal of Chemical Physics. ISSN 0021-9606. 163(3). doi: 10.1063/5.0274736. Fulltekst i vitenarkiv Vis sammendrag

  The paper investigates the basis set incompleteness errors of the Hartree–Fock energies for molecules in extreme magnetic field strengths up to 5 B0 (≈106 T), considering electronic state, geometric structure, and orientation of the molecule with respect to the magnetic field. We compare the results from finite-field calculations using uncontracted correlation-consistent basis sets with the fully numerical solution using multiresolution analysis on the He atom and its dimer, as well as the methylidyne radical and the water molecule. Standard uncontracted aug-cc-pVQZ basis sets are generally reliable up to B = 0.2 B0. Between B = 0.5 B0 and 1.0 B0, care must be taken as the ground state switches to states with high multiplicities, for which standard basis sets have not been optimized. Beyond B = 1.0 B0, the angular and state dependence of these basis sets becomes too large and too unsystematic for results to be considered reliable.

• Burger, Sophia; Stopkowicz, Stella & Gauss, Jürgen (2025). Cholesky Decomposition and the Second-Derivative Two-Electron Integrals Required for the Computation of Magnetizabilities Using Gauge-Including Atomic Orbitals. Journal of Physical Chemistry A. ISSN 1089-5639. 129(2), s. 623–632. doi: 10.1021/acs.jpca.4c07421. Fulltekst i vitenarkiv Vis sammendrag

  The computation of magnetizability tensors using gauge-including atomic orbitals is discussed in the context of Cholesky decomposition (CD) for the two-electron repulsion integrals with a focus on the involved doubly differentiated integrals. Three schemes for their handling are suggested: the first exploits the density fitting (DF) aspect of Cholesky decomposition, the second uses expressions obtained by differentiating the CD expression for the unperturbed two-electron integrals, while the third addresses the issue that the first two schemes are not able to represent the doubly differentiated integrals with arbitrary accuracy. This scheme uses a separate Cholesky decomposition for the cross terms in the doubly differentiated two-electron integrals. Test calculations reveal that all three schemes are able to represent the integrals with similar accuracy and yield indistinguishable results for the values of the computed magnetizability tensor elements. Thus, we recommend our first scheme which has the lowest computational cost for routine computations. The applicability of our CD schemes is further shown in large-scale Hartree–Fock calculations of the magnetizability tensor of coronene (C24H12) with a doubly polarized triple-ζ basis consisting of 684 basis functions.

• Kitsaras, Marios-Petros; Grazioli, Laura & Stopkowicz, Stella (2024). The approximate coupled-cluster methods CC2 and CC3 in a finite magnetic field. Journal of Chemical Physics. ISSN 0021-9606. 160(9). doi: 10.1063/5.0189350. Fulltekst i vitenarkiv Vis sammendrag

  In this paper, we report on the implementation of CC2 and CC3 in the context of molecules in finite magnetic fields. The methods are applied to the investigation of atoms and molecules through spectroscopic predictions and geometry optimizations for the study of the atmosphere of highly magnetized White Dwarf stars. We show that ground-state finite-field (ff) CC2 is a reasonable alternative to CCSD for energies and, in particular, for geometrical properties. For excited states, ff-CC2 is shown to perform well for states with predominant single-excitation character. Yet, for cases in which the excited state wavefunction has double-excitation character with respect to the reference, ff-CC2 can easily lead to completely unphysical results. Ff-CC3, however, is shown to reproduce the CCSDT behavior very well and enables the treatment of larger systems at a high accuracy.

• Grazioli, Laura; Schleicher, Luca T.; Stopkowicz, Stella & Gauss, Jürgen (2024). Theoretical prediction of closed-shell paramagnetism for scandium and yttrium hydride. Journal of Computational Chemistry. ISSN 0192-8651. doi: 10.1002/jcc.27305. Fulltekst i vitenarkiv
• Blaschke, Simon; Kitsaras, Marios-Petros & Stopkowicz, Stella (2024). Finite-field Cholesky decomposed coupled-cluster techniques (ff-CD-CC): theory and application to pressure broadening of Mg by a He atmosphere and a strong magnetic field. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. doi: 10.1039/d4cp03103b. Fulltekst i vitenarkiv Vis sammendrag

  For the interpretation of spectra of magnetic stellar objects such as magnetic white dwarfs (WDs), highly accurate quantum chemical predictions for atoms and molecules in finite magnetic field are required. Especially the accurate description of electronically excited states and their properties requires established methods such as those from coupled-cluster (CC) theory. However, respective calculations are computationally challenging even for medium-sized systems. Cholesky decomposition (CD) techniques may be used to alleviate memory bottlenecks. In finite magnetic field computations, the latter are increased due to the reduction of permutational symmetry within the electron-repulsion-integrals (ERIs) as well as the need for complex-valued data types. CD enables a memory-efficient, approximate description of the ERIs with rigorous error control and thus the treatment of larger systems at the CC level becomes feasible. In order to treat molecules in a finite magnetic field, we present in this work the working equations of the left and right-hand side equations for finite field (ff)-EOM-CD-CCSD for various EOM flavours as well as for the approximate ff-EOM-CD-CC2 method. The methods are applied to the study of the modification of the spectral lines of a magnesium transition by a helium atmosphere that can be found on magnetic WD stars.

• Blaschke, Simon; Stopkowicz, Stella & Pausch, Ansgar (2024). Efficient approximate screening techniques for integrals over London atomic orbitals. Journal of Chemical Physics. ISSN 0021-9606. 161(2). doi: 10.1063/5.0217246. Fulltekst i vitenarkiv Vis sammendrag

  Efficient integral screening techniques are essential for the investigation of extended molecular structures. This work presents a critical assessment of well-established approximate screening techniques and extends them for integrals over London atomic orbitals, which are required in the presence of strong, external magnetic fields. Through the examination of helium clusters in such extreme environments, we demonstrate that seemingly straightforward extensions of field-free screening techniques as proposed in the recent literature can lead to significant errors. To rectify this, we propose two alternative screening techniques that lead to the desired speedups while still maintaining strict error control.

• Monzel, Laurenz & Stopkowicz, Stella (2024). Diagrams in Polaritonic Coupled Cluster Theory. Journal of Physical Chemistry A. ISSN 1089-5639. doi: 10.1021/acs.jpca.4c04389. Fulltekst i vitenarkiv
• Zhang, Chaoqun; Lipparini, Filippo; Stopkowicz, Stella; Gauss, Jürgen & Cheng, Lan (2023). Cholesky Decomposition-Based Implementation of Relativistic Two-Component Coupled-Cluster Methods for Medium-Sized Molecules. Journal of Chemical Theory and Computation. ISSN 1549-9618. 20(2), s. 787–798. doi: 10.1021/acs.jctc.3c01236. Fulltekst i vitenarkiv
• Ofstad, Benedicte; Wibowo-Teale, Meilani; Kristiansen, Håkon Emil; Aurbakken, Einar; Kitsaras, Marios-Petros & Schøyen, Øyvind Sigmundson [Vis alle 12 forfattere av denne artikkelen] (2023). Magnetic optical rotation from real-time simulations in finite magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 159(20), s. 1–14. doi: 10.1063/5.0171927. Fulltekst i vitenarkiv Vis sammendrag

  We present a numerical approach to magnetic optical rotation based on real-time time-dependent electronic-structure theory. Not relying on perturbation expansions in the magnetic-field strength, the formulation allows us to test the range of validity of the linear relation between the rotation angle per unit path length and the magnetic-field strength that was established empirically by Verdet 160 years ago. Results obtained from time-dependent coupled-cluster and time-dependent current density-functional theory are presented for the closed-shell molecules H2, HF, and CO in magnetic fields up to 55 kT at standard temperature and pressure conditions. We find that Verdet's linearity remains valid up to roughly 10-20 kT, above which significant deviations from linearity are observed. Among the three current density-functional approximations tested in this work, the current-dependent Tao-Perdew-Staroverov–Scuseria hybrid functional performs the best in comparison with time-dependent coupled-cluster singles and doubles results for the magnetic optical rotation.

• Hollands, M.A.; Stopkowicz, Stella; Kitsaras, M.-P.; Hampe, F.; Blaschke, S. & Hermes, J.J. (2023). A DZ white dwarf with a 30 MG magnetic field. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 520(3), s. 3560–3575. doi: 10.1093/mnras/stad143. Fulltekst i vitenarkiv Vis sammendrag

  ABSTRACT Magnetic white dwarfs with field strengths below 10 MG are easy to recognize since the Zeeman splitting of spectral lines appears proportional to the magnetic field strength. For fields ≳100 MG, however, transition wavelengths become chaotic, requiring quantum-chemical predictions of wavelengths and oscillator strengths with a non-perturbative treatment of the magnetic field. While highly accurate calculations have previously been performed for hydrogen and helium, the variational techniques employed become computationally intractable for systems with more than three to four electrons. Modern computational techniques, such as finite-field coupled-cluster theory, allow the calculation of many-electron systems in arbitrarily strong magnetic fields. Because around 25 per cent of white dwarfs have metal lines in their spectra, and some of those are also magnetic, the possibility arises for some metals to be observed in very strong magnetic fields, resulting in unrecognizable spectra. We have identified SDSS J114333.48+661531.83 as a magnetic DZ white dwarf, with a spectrum exhibiting many unusually shaped lines at unknown wavelengths. Using atomic data calculated from computational finite-field coupled-cluster methods, we have identified some of these lines arising from Na, Mg, and Ca. Surprisingly, we find a relatively low field strength of 30 MG, where the large number of overlapping lines from different elements make the spectrum challenging to interpret at a much lower field strength than for DAs and DBs. Finally, we model the field structure of SDSS J1143+6615 finding the data are consistent with an offset dipole.

• Verplancke, Hendrik; Diefenbach, Martin; Lienert, Jonas N.; Ugandi, Mihkel; Kitsaras, Marios-Petros & Roemelt, Michael [Vis alle 8 forfattere av denne artikkelen] (2023). Another Torture Track for Quantum Chemistry: Reinvestigation of the Benzaldehyde Amidation by Nitrogen-Atom Transfer from Platinum(II) and Palladium(II) Metallonitrenes. Israel Journal of Chemistry (IJC). ISSN 0021-2148. doi: 10.1002/ijch.202300060. Fulltekst i vitenarkiv Vis sammendrag

  Abstract We showcase here a dramatic failure of CCSD(T) theory that originates from the pronounced multi‐reference character of a key intermediate formed in the benzaldehyde amidation by N‐atom transfer from Pd(II) and Pt(II) metallonitrenes studied recently in combined experimental and theoretical work. For detailed analysis we devised a minimal model system, for which we established reliable reference energies based on approximate full configuration interaction theory, to assess the performance of single‐reference coupled‐cluster theory up to the CCSDTQ(P) excitation level. While RHF‐based CCSD(T) theory suffered dramatic errors, in one case exceeding 220 kcal mol −1 , we show that the use of broken‐symmetry (BS) or Kohn‐Sham (KS) orbital references yields substantially improved CCSD(T) results. Further, the EOM‐SF‐CCSD(T)(a)* approach met the reference data with excellent accuracy. We applied the KS‐CCSD(T*)‐F12b variant as high‐level part of an ONIOM(KS‐CC:DFT) scheme to reinvestigate the reactivity of the full Pt(II) and Pd(II) metallonitrenes. The revised reaction pathway energetics provide a detailed mechanistic rationale for the experimental observations.

• 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), s. 214112-1–214112-9. doi: 10.1063/1.5093396. Fulltekst i vitenarkiv
• Reimann, Sarah; Borgoo, Alex; Austad, Jon; Tellgren, Erik; Teale, Andrew Michael & Helgaker, Trygve [Vis alle 7 forfattere av denne artikkelen] (2019). Kohn–Sham energy decomposition for molecules in a magnetic field. Molecular Physics. ISSN 0026-8976. 117(1), s. 97–109. doi: 10.1080/00268976.2018.1495849. Fulltekst i vitenarkiv
• Epifanovsky, Evgeny; Klein, Kerstin; Stopkowicz, Stella; Gauss, Jürgen & Krylov, Anna I. (2015). Spin-orbit couplings within the equation-of-motion coupled-cluster framework: Theory, implementation, and benchmark calculations. Journal of Chemical Physics. ISSN 0021-9606. 143:064102(6). doi: 10.1063/1.4927785. Fulltekst i vitenarkiv
• 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), s. 18834–18842. doi: 10.1039/c5cp02682b. Fulltekst i vitenarkiv
• 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. Fulltekst i vitenarkiv
• Furness, James W.; Verbeke, Joachim; Tellgren, Erik; Stopkowicz, Stella; Ekström, Ulf Egil & Helgaker, Trygve [Vis alle 7 forfattere av denne artikkelen] (2015). Current density functional theory using meta-generalized gradient exchange-correlation functionals. Journal of Chemical Theory and Computation. ISSN 1549-9618. 11(9), s. 4169–4181. doi: 10.1021/acs.jctc.5b00535. Fulltekst i vitenarkiv
• Stopkowicz, Stella & Gauss, Jürgen (2014). Revised values for the nuclear quadrupole moments of S33 and S35. Physical Review A. Atomic, Molecular, and Optical Physics (PRA). ISSN 1050-2947. 90(2). doi: 10.1103/PhysRevA.90.022507. Fulltekst i vitenarkiv
• Cheng, Lan; Stopkowicz, Stella & Gauss, Jürgen (2014). Analytic Energy Derivatives in Relativistic Quantum Chemistry. International Journal of Quantum Chemistry. ISSN 0020-7608. 114(17), s. 1108–1127. doi: 10.1002/qua.24636. Fulltekst i vitenarkiv
• Stopkowicz, Stella; Cheng, Lan; Harding, Michael E.; Puzzarini, Cristina & Gauss, Jürgen (2013). The bromine nuclear quadrupole moment revisited. Molecular Physics. ISSN 0026-8976. 111(9-11), s. 1382–1389. doi: 10.1080/00268976.2013.796072. Fulltekst i vitenarkiv
• Cheng, Lan; Stopkowicz, Stella & Gauss, Jürgen (2013). Spin-free Dirac-Coulomb calculations augmented with a perturbative treatment of spin-orbit effects at the Hartree-Fock level. Journal of Chemical Physics. ISSN 0021-9606. 139, s. 214114–214114. doi: 10.1063/1.4832739. Fulltekst i vitenarkiv
• Cheng, Lan; Stopkowicz, Stella; Stanton, John F. & Gauss, Jurgen (2012). The route to high accuracy in ab initio calculations of Cu quadrupole-coupling constants. Journal of Chemical Physics. ISSN 0021-9606. 137(22). doi: 10.1063/1.4767767. Fulltekst i vitenarkiv

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• Kitsaras, Marios-Petros; Grazioli, Laura & Stopkowicz, Stella (2024). Erratum: Publisher's Note: "The approximate coupled-cluster methods CC2 and CC3 in a finite magnetic field" [J. Chem. Phys. 160, 094112 (2024) (The Journal of chemical physics (2024) 160 9 PII: 079901 DOI: 10.1063/5.0189350). Journal of Chemical Physics. ISSN 0021-9606. 161(7). doi: 10.1063/5.0231676. Fulltekst i vitenarkiv
• 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. Fulltekst i vitenarkiv
• 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. Fulltekst i vitenarkiv
• Helgaker, Trygve; Borgoo, Alex; Dimitrova, Maria; Gauss, Jurgen; Hampe, Florian & Holzer, Christof [Vis alle 13 forfattere av denne artikkelen] (2018). Diamagnetism and Paramagnetism in Atoms and Molecules. Fulltekst i vitenarkiv
• Helgaker, Trygve; Borgoo, Alex; Gauss, Jurgen; Hampe, Florian; Klopper, Wim & Reimann, Sarah [Vis alle 9 forfattere av denne artikkelen] (2018). Closed-Shell Molecular Paramagnetism. Fulltekst i vitenarkiv
• Helgaker, Trygve; Borgoo, Alex; Gauss, Jurgen; Hampe, Florian; Klopper, Wim & Reimann, Sarah [Vis alle 9 forfattere av denne artikkelen] (2018). Closed-Shell Molecular Paramagnetism. Fulltekst i vitenarkiv
• Helgaker, Trygve; Borgoo, Alex; Gauss, Jurgen; Hampe, Florian; Klopper, Wim & Stopkowicz, Stella [Vis alle 8 forfattere av denne artikkelen] (2018). Closed-Shell Molecular Magnetism. Fulltekst i vitenarkiv
• Stopkowicz, Stella & Helgaker, Trygve (2014). Coupled-Cluster Theory for Molecules in Strong Magnetic Fields. Fulltekst i vitenarkiv Vis sammendrag

  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).

• Stopkowicz, Stella; Cheng, Lan & Gauss, Jurgen (2014). Accurate Determination of Nuclear Quadrupole Moments and Quadrupole-Coupling Constants. Fulltekst i vitenarkiv Vis sammendrag

  Nuclear quadrupole moments provide information about the charge distribution of a given nucleus. Their interaction with the electric-field gradient at that nucleus leads to nuclear-quadrupole cou- plings that can provide fingerprint information about the local chemical environment of the given nucleus which is observable in various spectroscopic techniques like rotational, NMR or Mössbauer spectroscopy. The reliable determination of nuclear quadrupole moments as well as quadrupole-coupling constants requires the accurate computation of electric-field gradients. The latter requires an accurate description of the electron density in the vicinity of the nucleus and is challenging due to slow basis-set convergence as well as the importance of electron correlation and relativistic effects. In this talk, we report on a protocol for the highly accurate determination of electric-field gradients based on large-scale coupled-cluster calculations with additional consideration of scalar-relativistic and spin-orbit effects. We apply this protocol • to revise the values for the bromine and sulfur quadrupole moments[1, 2] based on an analysis of experimental quadrupole couplings for HBr as well as CS and SiS, respectively, • for the accurate calculation of copper quadrupole-coupling constants[3], • to determine within a joint experimental and theoretical investigation the hyperfine structure in the rotational spectrum of CH2FI[4]. [1] S. Stopkowicz et al., Mol. Phys., 111, 1382 (2013). [2] S. Stopkowicz and J. Gauss, in preparation. [3] L. Cheng et al., J. Chem. Phys., 137, 224302 (2012). [4] C. Puzzarini et al., J. Chem. Phys., 134, 174312 (2011).

• Stopkowicz, Stella (2013). Does Direct Perturbation Theory converge? Fulltekst i vitenarkiv
• Stopkowicz, Stella & Helgaker, Trygve (2013). Correlated molecular energies in strong magnetic fields. Fulltekst i vitenarkiv
• Stopkowicz, Stella (2013). Does Direct Perturbation Theory converge? Fulltekst i vitenarkiv

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