Self-consistent M{o}ller-Plesset Perturbation Theory For Excited States

In quantum chemistry, obtaining a system's mean-field solution and incorporating electron correlation in a post Hartree-Fock (HF) manner comprise one of the standard protocols for ground-state calculations. In principle, this scheme can also describe excited states but is not widely used at present, primarily due to the difficulty of locating the mean-field excited states. With recent developments in excited-state orbital relaxation, self-consistent excited-state solutions can now be located routinely at various levels of theory. In this work, we explore the possibility of correcting HF excited states using M{\o}ller-Plesset perturbation theory to the second order. Among various PT2 variants, we find that the restricted open-shell MP2 (ROMP2) gives excitation energies comparable to the best density functional theory results, delivering $\sim 0.2$ eV mean unsigned error over a wide range of single-configuration state function excitations, at only non-iterative $O(N^5)$ computational scaling.