In experiments using high-energy radiation (such as x-ray spectroscopy, EXAFS,
NEXAFS, XAS, XES, RIXS, REXS, etc.) core electrons can be ionized or excited to
low-lying virtual orbitals. There are several ways to compute ionization or
excitation energies of core electrons in Q-Chem. Standard approaches for
excited and ionized states need to be modified to tackle core-level states,
because these states have very high energies and are embedded in the ionization
continuum, i.e., they are Feshbach resonances.
1076
J. Chem. Phys.
(2017),
147,
pp. 014107.
Link