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Abstract: Core-to-valence transitions probed by x-ray absorption spectroscopy provide detailed information about the valence virtual orbitals. This is particularly valuable for molecules and materials with open-shell ground states, which often exhibit (quasi-)degenerate frontier molecular orbitals. The extended configuration-interaction singles (XCIS) method is a simple, variational, and size-consistent wave function approach that augments the conventional CIS space with a selected set of doubly substituted determinants. This extension enables the recovery of spin-pure excited states starting from a restricted open-shell Hartree–Fock (ROHF) reference. By construction, XCIS removes the severe spin contamination that affects unrestricted CIS in open-shell systems and delivers improved accuracy compared to ROHF-based CIS. In this work, we present an implementation of XCIS within the core/valence separation (CVS) approximation, which limits the active orbital space to core orbitals relevant for core-to-valence excitations. The resulting XCIS-CVS approach is applied to K-edge transitions in various open-shell systems, including 3d transition-metal complexes, yielding semi-quantitative agreement with experiment for both K-edge and pre-edge orbital splittings.
About the Presenter: Avik Ojha is a Ph.D. researcher in theoretical and computational chemistry at The Ohio State University, working in Dr. John M. Herbert’s research group. His research focuses on electronic structure methods, excited-state theory, and X-ray absorption spectroscopy, with an emphasis on implementing methods to model core-level spectra for open-shell molecules. Avik has contributed many useful features to Q-Chem over the last several years, including the CVS-XCIS implementation released last month as part of Q-Chem 6.4. His broader interests include developing new theoretical methods and using data-driven approaches to predict electronic and spectroscopic properties of large open-shell and transition-metal complexes.