A Promising Intersection of Excited-State-Specific Methods from Quantum Chemistry and Quantum Monte Carlo

Abstract: We present a discussion of recent progress in excited-state-specific quantum chemistry and quantum Monte Carlo alongside a demonstration of how a combination of methods from these two fields can offer reliably accurate excited state predictions across singly excited, doubly excited, and charge transfer states. Both of these fields have seen important advances supporting excited state simulation in recent years, including the introduction of more effective excited-state-specific optimization methods, improved handling of complicated wave function forms, and ways of explicitly balancing the quality of wave functions for ground and excited states. To emphasize the promise that exists at this intersection, we provide demonstrations using a combination of excited-state-specific complete active space self-consistent field theory, selected configuration interaction, and state-specific variance minimization. These demonstrations show that combining excited-state-specific quantum chemistry and variational Monte Carlo can be more reliably accurate than either equation of motion coupled cluster theory or multi-reference perturbation theory, and that it can offer new clarity in cases where existing high-level methods do not agree.