Academic Interest(s)
I am interested in the time-evolution of quantum many-body problems. Specifically I solve the time-dependent Schrödinger equation for electronic quantum systems subject to time-dependent external fields. To achieve this within the lifetime of the universe I employ a plethora of so-called ab-initio methods such as coupled-cluster theory, configuration interaction, Hartree-Fock and the likes. These methods lets us study physical processes in real-time in a fashion resembling actual experiments.
Courses taught
Background
- MSc Computational Physics, University of Oslo (2019)
- BSc Physics, University of Oslo (2017)
- BSc Informatics, University of Oslo (2017)
Tags:
Computational Physics,
Many-body Physics,
Computational quantum mechanics
Publications
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Aurbakken, Einar; Ofstad, Benedicte; Kristiansen, Håkon Emil; Schøyen, Øyvind Sigmundson; Kvaal, Simen & Sørensen, Lasse Kragh
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(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.
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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.
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Ofstad, Benedicte; Wibowo-Teale, Meilani; Kristiansen, Håkon Emil; Aurbakken, Einar; Kitsaras, Marios-Petros & Schøyen, Øyvind Sigmundson
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(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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Bathen, Marianne Etzelmüller; Hebnes, Oliver Lerstøl; Schøyen, Øyvind Sigmundson; Winther-Larsen, Sebastian Gregorius; Vines, Lasse & Hjorth-Jensen, Morten
(2022).
Predicting Solid State Material Platforms for Quantum Technologies.
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Published
Dec. 12, 2019 1:00 PM
- Last modified
May 9, 2022 12:47 PM