Publications by authors named "S I Kvaal"

52 Publications

Lower Semicontinuity of the Universal Functional in Paramagnetic Current-Density Functional Theory.

J Phys Chem Lett 2021 Feb 1;12(5):1421-1425. Epub 2021 Feb 1.

Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway.

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.
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http://dx.doi.org/10.1021/acs.jpclett.0c03422DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7883387PMC
February 2021

Interpretation of Coupled-Cluster Many-Electron Dynamics in Terms of Stationary States.

J Chem Theory Comput 2021 Jan 18;17(1):388-404. Epub 2020 Dec 18.

Department of Physics, University of Oslo, N-0315 Oslo, Norway.

We demonstrate theoretically and numerically that laser-driven many-electron dynamics, as described by bivariational time-dependent coupled-cluster (CC) theory, may be analyzed in terms of stationary-state populations. Projectors heuristically defined from linear response theory and equation-of-motion CC 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 He and Be atoms and for 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 full configuration interaction (FCI) theory, provided that 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 deteriorates. 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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http://dx.doi.org/10.1021/acs.jctc.0c00977DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7808707PMC
January 2021

Photodynamic therapy of oral lichen planus.

Photochem Photobiol Sci 2020 Oct;19(10):1271-1279

University of Oslo, Faculty of Dentistry, Institute of Clinical Dentistry, Department of Oral Surgery and Oral Medicine, Oslo, Norway.

Oral lichen planus (OLP) is a chronic inflammatory and immune-mediated disease affecting the oral mucosa. OLP presents with asymptomatic, lacelike white stripes and/or symptomatic red, ulcerated mucous membranes. Eating, drinking and oral hygiene procedures may be painful resulting in reduced quality of life (QOL). The histopathological picture is a zone of cellular infiltrate, mainly CD8+ cells, in the superficial layer of the connective tissue and signs of liquefaction degeneration of the basal membrane. Conventional treatment is corticosteroids. Local and systemic side effects are common, and patients may develop drug resistance. The intention with this article is to demonstrate the heterogeneity in photodynamic therapy (PDT) of OLP. A search in PubMed, Embase (Ovid) and Medline (Ovid) identified seventeen clinical studies investigating PDT of OLP. Only five were randomised controlled studies and the study groups varied from 5 to 50 patients. Five different photosensitisers or precursors were tested. Both broadband spectrum lamps, lasers and light-emitting-diodes (LEDs), with wavelengths from 420 nm to 682 nm, were used. The number of treatment sessions varied from one to ten. The patients were followed up for 0 to 48 months, but in thirteen studies the post treatment observation time was ≤6 months. Single arm studies demonstrated improvement of OLP except in one study. In all controlled studies except for one, PDT was superior or equal to conventional treatment. The majority of patients experienced a slight burning sensation during light activation, but no serious adverse events were reported. Only few studies examined the effect of PDT on a cellular level.
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http://dx.doi.org/10.1039/d0pp00249fDOI Listing
October 2020

A state-specific multireference coupled-cluster method based on the bivariational principle.

J Chem Phys 2020 Jul;153(2):024106

Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway.

A state-specific multireference coupled-cluster (MRCC) method based on Arponen's bivariational principle is presented, the bivar-MRCC method. The method is based on single-reference theory and therefore has a relatively straightforward formulation and modest computational complexity. The main difference from established methods is the bivariational formulation, in which independent parameterizations of the wave function (ket) and its complex conjugate (bra) are made. Importantly, this allows manifest multiplicative separability of the state (exact in the extended bivar-MRECC version of the method and approximate otherwise), and additive separability of the energy, while preserving polynomial scaling of the working equations. A feature of the bivariational principle is that the formal bra and ket references can be included as bivariational parameters, which eliminates much of the bias toward the formal reference. A pilot implementation is described, and extensive benchmark calculations on several standard problems are performed. The results from the bivar-MRCC method are comparable to established state-specific multireference methods. Considering the relative affordability of the bivar-MRCC method, it may become a practical tool for non-experts.
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http://dx.doi.org/10.1063/5.0009429DOI Listing
July 2020

Numerical stability of time-dependent coupled-cluster methods for many-electron dynamics in intense laser pulses.

J Chem Phys 2020 Feb;152(7):071102

Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway.

We investigate the numerical stability of time-dependent coupled-cluster theory for many-electron dynamics in intense laser pulses, comparing two coupled-cluster formulations with full configuration interaction theory. Our numerical experiments show that orbital-adaptive time-dependent coupled-cluster doubles (OATDCCD) theory offers significantly improved stability compared with the conventional Hartree-Fock-based time-dependent coupled-cluster singles-and-doubles (TDCCSD) formulation. The improved stability stems from greatly reduced oscillations in the doubles amplitudes, which, in turn, can be traced to the dynamic biorthonormal reference determinants of OATDCCD theory. As long as these are good approximations to the Brueckner determinant, OATDCCD theory is numerically stable. We propose the reference weight as a diagnostic quantity to identify situations where the TDCCSD and OATDCCD theories become unstable.
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http://dx.doi.org/10.1063/1.5142276DOI Listing
February 2020