Abstract
The use of spin-dependent orbital energies is proposed in the multireference Møller–Plesset perturbation theory. In this method, a single canonical Fock orbital set is used for both alpha and beta electrons, while the orbital energies are dependent on the spin of the electron that occupies the orbital. The method is tested on the energy splitting between states of different spin in the CH₂, CF₂, CO, O₂, N₂⁺, and o-benzyne molecules. The results agree well with available experimental data: the deviations are within 4 kcal/mol in all cases considered.