The Variationally Orbital-Adapted Configuration Interaction Singles (VOA-CIS) Approach to Electronically Excited States.

For chemically accurate excited state energies, one is forced to include electron-electron correlation at a level of theory significantly higher than configuration interaction singles (CIS). Post-CIS corrections do exist, but most often, if they are computationally inexpensive, these methods rely on perturbation theory. At the same time, inexpensive variational post-CIS methods would be ideal since modeling electronic relaxation usually requires globally smooth potential energy surfaces (PESs) and there will inevitably be regions of near electronic degeneracy. With that goal in mind, we now present a new method entitled variationally orbital adapted CIS (VOA-CIS). On the one hand, we show that in the ground-state geometry, VOA-CIS performs comparably to CIS(D) at predicting relative excited state energies. On the other hand, far beyond CIS(D) or any other perturbative method, VOA-CIS correctly rebalances the energy of charge-transfer (CT) states versus non-CT states, while simultaneously producing smooth PESs-including the important case of avoided crossings. In fact, through localized diabatization of VOA-CIS excited states, one can find a set of reasonable diabatic states modeling CT chemical dynamics. After significant benchmarking, we are now confident VOA-CIS and VOA-CIS-like methods should play a major role in future excited state calculations.