7.13 Core Ionization Energies and Core-Excited States

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. ¹⁰⁴⁷ Sadybekov A., Krylov A. I.
J. Chem. Phys.
(2017), 147, pp. 014107. Link