Calculating Core-Excited States and X-ray Absorption Spectroscopy with Non-Orthogonal Configuration Interaction with Single Substitutions (NOCIS) Methods

• NOCIS is a non-orthogonal configuration interaction (NOCI) method specifically applied to core-excited state calculations. It is identical to the static exchange (STEX) method in the case of single core hole reference.
• In NOCIS, orbital relaxation is considered through ROHF calculations, and static correlation between the  non-orthogonal core-hole references is included through CI. Dynamic correlation is not included in NOCI, but may be included through perturbation.
• Not like the standard NOCI method, NOCIS is designed as a blackbox theory which includes a precisely defined set of determinants.
• NOCIS is spin-pure, size consistent, and maintains spatial symmetry.
• NOCIS can predict accurate core excitation energies comparable to the results from high-level methods such as ADC, but has a much lower computational scaling.
• As an approximation, one-center (1C)-NOCIS greatly reduces the computational cost of NOCIS.