Einar Aurbakken

• Aurbakken, Einar; Ofstad, Benedicte; Kristiansen, Håkon Emil; Schøyen, Øyvind Sigmundson; Kvaal, Simen & Sørensen, Lasse Kragh [Show all 8 contributors for this article] (2024). Transient spectroscopy from time-dependent electronic-structure theory without multipole expansions. Physical Review A (PRA). ISSN 2469-9926. 109(1). doi: 10.1103/PhysRevA.109.013109. Full text in Research Archive Show summary

  Based on the work done by an electromagnetic field on an atomic or molecular electronic system, a general gauge invariant formulation of transient absorption spectroscopy is presented within the semi-classical approximation. Avoiding multipole expansions, a computationally viable expression for the spectral response function is derived from the minimal-coupling Hamiltonian of an electronic system interacting with one or more laser pulses described by a source-free, enveloped electromagnetic vector potential. With a fixed-basis expansion of the electronic wave function, the computational cost of simulations of laser-driven electron dynamics beyond the dipole approximation is the same as simulations adopting the dipole approximation. We illustrate the theory by time-dependent configuration interaction and coupled-cluster simulations of core-level absorption and circular dichroism spectra.

• Ofstad, Benedicte; Wibowo-Teale, Meilani; Kristiansen, Håkon Emil; Aurbakken, Einar; Kitsaras, Marios-Petros & Schøyen, Øyvind Sigmundson [Show all 12 contributors for this article] (2023). Magnetic optical rotation from real-time simulations in finite magnetic fields. Journal of Chemical Physics. ISSN 0021-9606. 159(20), p. 1–14. doi: 10.1063/5.0171927. Full text in Research Archive Show summary

  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.

• Ofstad, Benedicte; Kristiansen, Håkon Emil; Aurbakken, Einar; Schøyen, Øyvind Sigmundson; Kvaal, Simen & Pedersen, Thomas Bondo (2023). Adiabatic extraction of nonlinear optical properties from real-time time-dependent electronic-structure theory. Journal of Chemical Physics. ISSN 0021-9606. 158(15). doi: 10.1063/5.0145521. Full text in Research Archive Show summary

  Real-time simulations of laser-driven electron dynamics contain information about molecular optical properties through all orders in response theory. These properties can be extracted by assuming convergence of the power series expansion of induced electric and magnetic multipole moments. However, the accuracy relative to analytical results from response theory quickly deteriorates for higher-order responses due to the presence of high-frequency oscillations in the induced multipole moment in the time domain. This problem has been ascribed to missing higher-order corrections. We here demonstrate that the deviations are caused by nonadiabatic effects arising from the finite-time ramping from zero to full strength of the external laser field. Three different approaches, two using a ramped wave and one using a pulsed wave, for extracting electrical properties from real-time time-dependent electronic-structure simulations are investigated. The standard linear ramp is compared to a quadratic ramp, which is found to yield highly accurate results for polarizabilities, and first and second hyperpolarizabilities, at roughly half the computational cost. Results for the third hyperpolarizability are presented along with a simple, computable measure of reliability.

• Ofstad, Benedicte; Aurbakken, Einar; Schøyen, Øyvind Sigmundson; Kristiansen, Håkon Emil; Kvaal, Simen & Pedersen, Thomas Bondo (2023). Time-dependent coupled-cluster theory. Wiley Interdisciplinary Reviews. Computational Molecular Science. ISSN 1759-0876. 13(5). doi: 10.1002/wcms.1666. Full text in Research Archive Show summary

  Recent years have witnessed an increasing interest in time-dependent coupled-cluster (TDCC) theory for simulating laser-driven electronic dynamics in atoms and molecules, and for simulating molecular vibrational dynamics. Starting from the time-dependent bivariational principle, we review different flavors of single-reference TDCC theory with either orthonormal static, orthonormal time-dependent, or biorthonormal time-dependent spin orbitals. The time-dependent extension of equation-of-motion coupled-cluster theory is also discussed, along with the applications of TDCC methods to the calculation of linear absorption spectra, linear and low-order nonlinear response functions, highly nonlinear high harmonic generation spectra and ionization dynamics. In addition, the role of TDCC theory in finite-temperature many-body quantum mechanics is briefly described along with a few other application areas.

• 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.

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• Kristiansen, Håkon Emil & Aurbakken, Einar (2024). Pseudospectral discretization of the time-dependent Schrödinger equation. arXiv.org. ISSN 2331-8422. doi: 10.48550/arXiv.2404.04109.
• 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.
• 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. Universitetet i Oslo.

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