Perturbation corrections to Koopmans' theorem. II. A study of basis set variation

Abstract
Ordinary third‐order Rayleigh Schrödinger 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 H2O, CH2, NH3, H2CO, F2O, and CO. The results are compared to those obtained from a double‐zeta basis set.