Computational study of vertical ionization potentials using the ΔINDO + FOCI method

Abstract

The computational procedure devised by Segal has been employed to estimate ionization potentials by energy differences. In order to obtain similar accuracy with both open- and closed-shell species, a first-order configuration interaction (FOCI) calculation is performed after the SCF procedure for the open-shell system. In this work, the SCF scheme is based on the INDO approximation. In general, these ΔINDO + FOCI computations provide adequate interpretations of photo-electron spectra. For closed-shell parent molecules, comparison of the ionization potentials calculated by Koopmans' Theorem and those by the ΔSCF + FOCI method leads to an estimate of the electronic reorganization taking place on ionization. The results of the present calculations indicate that electronic rcargauization is insufficient to affect the ordering of the first few cationic states predicted by Koopmans' Theorem. The major exception to this occurs for the molecules H₂CO and F₂CO.