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.
1C-NOCIS and Experimetal Spectrum for the Carbon 1s X-ray Absorption of (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl
1C-NOCIS spectra for singlet (black) andtriplet (red) 4-Mercaptobenzoic acid