An SCF–MO–CNDO study of ionization potentials and orbital energies by the CNDO/BW theory

Vertical ionization potentials, calculated as the difference between the total energies of the ground states of the ion and its parent molecule, are compared with estimates obtained by use of Koopmans' theorem. For some molecules the differences between the two theoretical values are quite large, while for molecules where the molecular orbitals are constrained by symmetry, the two theoretical values are identical. Differences between calculated adiabatic and vertical ionization potentials are shown to agree with the qualitative features of photoelectron spectra. The best overall fit of theoretical and experimental ionization potentials is obtained with atomic parameters evaluated from Hinze and Jaffe valence-state data and resonance integrals dependent upon valence-state ionization potentials. The theoretical predictions are generally superior to those of CNDO/2 or Hartree–Fock calculations.