Multireference X-Ray Emission and Absorption Spectroscopy calculations from Monte Carlo Configuration Interaction

TL;DR

This paper introduces a Monte Carlo configuration interaction method to accurately compute core-hole states for X-ray emission and absorption, comparing results with experimental data and other computational methods.

Contribution

The paper adapts Monte Carlo configuration interaction for core-hole state calculations in X-ray spectroscopy, providing a new approach for multireference systems.

Findings

Monte Carlo CI results agree with EOM-CCSD for emission

Results match experimental absorption data

Wavefunction multireference character quantified

Abstract

We adapt the method of Monte Carlo configuration interaction to calculate core-hole states and use this for the computation of X-ray emission and absorption values. We consider CO, CH$_{4}$, NH$_{3}$, H$_{2}$O, HF, HCN, CH$_{3}$OH, CH$_{3}$F, HCl and NO using a 6-311G** basis. We also look at carbon monoxide with a stretched geometry and discuss the dependence of its results on the cutoff used. The Monte Carlo configuration interaction results are compared with EOM-CCSD values for X-ray emission and with experiment for X-ray absorption. Oscillator strengths are also computed and we quantify the multireference nature of the wavefunctions to suggest when approaches based on a single reference would be expected to be successful.