Monte Carlo configuration interaction predictions for the electronic spectra of Ne, CH2, C2, N2, and H2O compared to full configuration interaction calculations

Singlet and triplet electronic excitation energies have been calculated for Ne, ${\text{CH}}_2$ , ${\text{C}}_2$ , ${\text{N}}_2$ , and ${\text{H}}_2\text{O}$ using the Monte Carlo configuration interaction (CI) method. We find that excitation energies can be predicted to within a few tens of meV of full CI (FCI) results using expansions consisting of only a few thousand configuration state functions as compared to the $O\left({10}^8\right)$ configurations occurring in the corresponding FCI expansions. The method provides a consistently accurate and balanced description of electronic excitations with accuracy for small molecular systems comparable to the equation-of-motion coupled cluster method with full triples.