A two-step Rayleigh-Schrödinger Brillouin-Wigner approach to transition energies

Perturbative methods are attractive to describe the electronic structure of molecular systems because of their low-computational cost and systematically improvable character. In this work, a two-step perturbative approach is introduced combining multi-state Rayleigh-Schrödinger (effective Hamiltonian theory) and state-specific Brillouin-Wigner schemes to treat degenerate configurations and yield an efficient evaluation of multiple energies. The first step produces model functions and an updated definition of the perturbative partitioning of the Hamiltonian. The second step inherits the improved starting point provided in the first step, enabling then faster processing of the perturbative corrections for each individual state. The here-proposed two-step method is exemplified on a model-Hamiltonian of increasing complexity.