Perturbation corrections to Koopmans' theorem. I. Double-zeta Slater-type-orbital basis

Ordinary third-order Rayleigh-Schrödinger perturbation theory is used to evaluate vertical ionization potentials of closed-shell molecules. The method is tested with double-zeta calculations on H2O, CH2, NH3, H2CO, and F2O. The results are compared with the ΔSCF method and discussed. On the basis of our calculations, we confirm the suggested alternative assignment for the photoelectron spectrum of F2O, instead of the original assignment based on Koopmans' theorem.