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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.

The method of ordinary third-order Rayleigh Schriidinger perturbation theory together with the geometric approximation is used to evaluate the vertical ionization potentials of HCI. H 2 S, H 3 P, H.Si, H3SiF. and H 3 CCI. The "I 1/2-zeta" STO basis set employed permits excellent agreement between theory and experiment to be attained. Computations on H 2 0, N 2 , and F2 using GTO basis sets show that the present method is equivalent to the many-body techniques employed by other workers.

Exact perturbation treatment of the basis set superposition correction J. Chem. Phys. 95, 6705 (1991); 10.1063/1.461540 The incomplete basis set problem. II. Perturbative corrections to coupled perturbed Hartree-Fock theory J. Chem. Phys. 69, 5050 (1978); 10.1063/1.436495Perturbation corrections to Koopmans' theorem. III. Extension to molecules containing Si, P, S, and Cl and comparison with other methods Ordinary third-order Rayleigh Schrodinger perturbation theory is used to evaluate vertical ionization potentials of closed-shell molecules. The variation in the accuracy of the method as the size of the basis set increases is investigated for water. From this study, the smallest basis compatible with reasonable accuracy is selected as the best compromise between cost and accuracy. This basis is used to calculate the vertical ionization potentials of H 2 0, CH 2 , NH 3 , H 2 CO, F 2 0, and CO. The results are compared to those obtained from a double-zeta basis set.E. f4s2p/2sj: This is Dunning's5,7 double-zeta Gaussian-type-orbital (GTO) basis set contracted from 9s5p/4s