Non-Orthogonal Configuration Interaction with Single Substitutions for Core-Excited States: An Extension to Doublet Radicals

• Published in: Journal of chemical theory and computation Vol. 15; no. 5; pp. 2966 - 2973
• Main Authors: Oosterbaan, Katherine J, White, Alec F, Head-Gordon, Martin
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14.05.2019
• Subjects: Configuration interaction; Density functional theory; Error reduction; Excitation; Mathematical analysis; Radicals; Time dependence
• ISSN: 1549-9618; 1549-9626
• DOI: 10.1021/acs.jctc.8b01259

In this paper, we present an open-shell extension of the non-orthogonal configuration interaction singles (NOCIS) method for the calculation of core-excited states, intended for peak assignment in XAS spectra of doublet radicals. This extension requires the consideration of additional configurations due to the singly occupied open-shell orbital, and the addition of essential orbital relaxation effects is found to provide a significant improvement on standard CIS, while maintaining the desirable properties of spin purity, variationality, and size consistency. We apply this method to the calculation of core excitations for several open-shell molecules and demonstrate that it performs competitively with other available methods, despite a lack of dynamic correlation. In particular, relative to CVS-ADC(2)-x, RMS error is reduced by a factor of 6 over usual orthogonal CIS and is comparable to time-dependent density functional theory with the best short-range corrected functionals.