Necessary many-body corrections in X-ray absorption spectra of open-shell metal compounds

X-ray absorption spectroscopy (XAS) provides direct access to local electronic structure information in materials and molecules and at interfaces. The technique benefits greatly from theoretical interpretation by modeling of atom-specific core-excited states. Within the context of final-state approaches to model core-level excitations, a full accounting of electronic response is advantageous for simulating accurate absorption spectra. The response to a core excitation spans a spectrum of phenomena ranging from simple polarization of the electron density around the excited atom to discrete charge transfer. Such strong charge transfer response is already evident in final-state calculations using density functional theory and has been known for decades within the context of ligand-field models. We provide a computational approach to correctly account for such effects in simulated spectra by spanning available many-electron determinants sampled from the final-state orbital perspective. Multiple examples are provided from transition metal oxides to organic molecules complexed with transition metal ions. Most illuminating is the importance of accounting for distinct response in each spin channel for collinear spin-polarized core-excited states, which leads to shifts in the energy of transitions and is an additional source of spectral broadening in addition to core-hole lifetime effects.