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Generalized Single Excitation Configuration Interaction: An Investigation into the Impact of the Inclusion of Non-Orthogonality on the Calculation of Core-Excited States

preprint
submitted on 05.12.2019 and posted on 13.12.2019 by Katherine J. Oosterbaan, Alec F. White, Diptarka Hait, Martin Head-Gordon
In this paper, we investigate different non-orthogonal generalizations of the configuration interaction with single substitutions (CIS) method for the calculation of core-excited states. Fully non-orthogonal CIS (NOCIS) has been described previously for singlets and doublets and this paper reports the extension to triplet molecules. In addition to NOCIS, we present a novel method, 1C-NOCIS(1C-NOCIS), for open-shell molecules which is intermediate between NOCIS and the static exchange approximation (STEX). We explore this hierarchy of spin-pure methods for singlet, doublet, and triplet molecules and conclude that, while NOCIS provides the best results and preserves the spatial symmetry of the wavefunction, 1C-NOCIS retains much of the accuracy of NOCIS at a dramatically reduced cost. For molecules with closed-shell ground states, STEX and 1C-NOCIS are identical.

Funding

This work is supported by the Director, Office of Science, Office of Basic Energy Sciences of the US Department of Energy under contract No. DE-AC02-05CH11231.

History

Email Address of Submitting Author

diptarka@berkeley.edu

Institution

University of California, Berkeley

Country

United States

ORCID For Submitting Author

0000-0003-1570-920X

Declaration of Conflict of Interest

There are no conflicts of interest to declare.

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