DFT/XCIS: A Kohn-Sham Approach to Extended Configuration-Interaction Singles, for Valence- and Core-Level Spectroscopy of Open-Shell Systems
Year: 2026
Type: preprint
Abstract: Core-to-valence transitions are sensitive probes of unoccupied valence orbitals and are especially informative for open-shell molecules and materials, where frontier molecular orbitals are often degenerate or nearly so. Accurate simulation of core-level spectra for open-shell molecules is challenging because methods such as time-dependent density functional theory (TDDFT) typically rely on a spin-unrestricted reference determinant, leading to spin contamination. A restricted open-shell reference state eliminates spin contamination in the ground state but excited states computed using the configuration-interaction singles (CIS) approach may yet exhibit significant spin contamination. This can be eliminated by extending the CIS excitation space to include a small number of doubly substituted determinants, in what has been called “extended” (X)CIS, and this approach affords a variational and size-consistent wave function for each excited state, free of spin contamination but also free of dynamical correlation. Here, we introduce the latter via density functional theory (DFT), using Kohn-Sham orbitals within the XCIS formalism and scaling the electron repulsion integrals to reduce double-counting of correlation effects. Reasonable valence excitation energies are obtained with the new DFT/XCIS approach, and combing it with core/valence separation (DFT/ XCIS-CVS) extends the method to core-level spectroscopy. Absolute excitation energies for K-edge transitions are improved, relative to conventional TDDFT, by means of an empirical shift in the 1s eigenvalues. Incorporation of dynamical correlation improves the accuracy of spectral splittings for open-shell systems, relative to the recently reported XCIS-CVS approach.
Source: ChemRxiv
Authors: Avik Kumar Ojha, John M. Herbert
Institution: The Ohio State University
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7204934113