Calculation of the energies of .pi.* negative ion resonance states by the use of Koopmans' theorem

The use of Koopmans'theorem (KT) for the calculation of scaled energies of π * negative ion resonance states has been investigatcd by ab initio molecular theory as a function of basis set. HF/D95v energies calculated for HF/6-31G-optimized geometries have been found to correlate with experimental electron attachment energies for 56 π * negative ion states in 39 alkenes, polyenes, and benzenoid hydrocarbons with a correlation coefficient of 0.989 and with average and largest absolute errors of 0.11 and 0.32 eV, respectively. An equally good correlation was obtained at the same basis set for MP2/6-31G * -optimized geometries. Various factors that influence the quality of KT correlations are analyzed