Perturbation corrections to Koopmans' theorem. IV. Energy‐shifted perturbation theory

Energy‐shifted perturbation theory is studied in connection with corrections to Koopmans' theorem. The first three vertical Ionization potentials of N2, C2H2, and HCN are computed with basis sets of double‐zeta Gaussian‐type orbitals and double‐zeta GTO plus polarization functions. The average absolute deviation of calculation ionization potentials from the observed values is found to be 0.4 eV, whether one uses Brillouin‐Wigner or Rayleigh‐Schrödinger perturbation theory.