Configuration Interaction Singles with Perturbative Doubles Correction [CIS(D)] theory can be loosely understood as an equivalent to second-order Møller-Plesset Perturbation Theory (MP2) for an excited state. The excitation energy at the CIS(D) level ∆E CIS(D) is, therefore, formally defined as the difference between the total energies of the excited E CIS(D) Ex and ground states E MP2