Multireference X-ray emission and absorption spectroscopy calculations from Monte Carlo configuration interaction

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 energies. We consider CO, $$\hbox {CH}_{4}$$CH4, $$\hbox {NH}_{3}$$NH3, $$\hbox {H}_{2}\hbox {O}$$H2O, HF, HCN, $$\hbox {CH}_{3}\hbox {OH}$$CH3OH, $$\hbox {CH}_{3}\hbox {F}$$CH3F, 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 or not.