Monte Carlo configuration interaction predictions for the electronic spectra of Ne, CH2, C2, N2, and H2 O compared to full configuration interaction calculations

Singlet and triplet electronic excitation energies have been calculated for Ne, CH2, C2, N2, and H2 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 (108) 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.