Extended configuration-interaction singles method with core/valence separation (XCIS-CVS): Core-level spectra of open-shell molecules

Core-to-valence transitions probed in x-ray absorption spectroscopy serve as reporters on the valence virtual orbitals. This is especially informative for molecules and materials with open-shell ground states, which typically feature (quasi-)degenerate frontier molecular orbitals. Extended configuration-interaction singles (XCIS) is a simple, variational, size-consistent wave function ansatz that incorporates a limited set of doubly substituted determinants into the usual CIS configuration space, in order to recover spin-pure excited states starting from a restricted open-shell Hartree-Fock (ROHF) ground state. It eliminates the severe spin contamination that plagues unrestricted CIS for open-shell systems, and also offers better accuracy as compared to an ROHF-based CIS calculation. Here, we report an implementation of XCIS based on the core/valence separation (CVS) approximation, which restricts the orbital active space to a few occupied orbitals associated with core-to-valence transitions. The XCIS-CVS method is applied to K-edge transitions in a variety of open-shell systems including 3d transition metal complexes. As compared to experiment, K-edge and pre-edge orbital splittings are reproduced semi-quantitatively.