2025/11/19 by Avik Kumar Ojha, John M. Herbert · 1 voice · 1 citation
Materials Science · Chemistry · Physics and Astronomy · #Magnetism in coordination complexes #Organometallic Complex Synthesis and Catalysis #Advanced Chemical Physics Studies
paper · doi:10.1021/acs.jctc.5c01578
openalex publication_date 2025/11/19 · openalex created_date 2025/11/19 · openalex updated_date 2026/05/16
Spectroscopic core-to-valence transitions serve as reporters on the valence virtual orbitals, which is especially informative for molecules and materials with open-shell ground states that feature (quasi-)degenerate frontier molecular orbitals. Excited states of open-shell molecules are difficult to model using methods based on single excitations only, a category that includes time-dependent density functional theory, due to severe spin contamination (in both ground and excited states) when a spin-unrestricted reference determinant is used. Extended configuration-interaction singles (XCIS) is a simple, variational, and size-consistent wave function ansatz that augments the usual CIS excitation space with a limited set of doubly substituted determinants in order to recover spin-pure excited states starting from a restricted open-shell Hartree–Fock (ROHF) ground state. XCIS eliminates spin contamination and offers better accuracy as compared to ROHF-based CIS. 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 so that core-to-valence transitions can be simulated efficiently. In applications of XCIS-CVS to X-ray transitions in a variety of open-shell systems, including 3d transition metal complexes, we find that both K-edge and pre-edge orbital splittings are reproduced semiquantitatively as compared to experiment.