Extension of Koopmans’ theorem. IV. Ionization potentials from correlated wavefunctions for molecular fluorine

Uncorrelated (Hartree–Fock) ab initio calculations have proven unable to predict the energy ordering of 3σg and 1πu molecular orbitals as observed in electron spectroscopy experiments on fluorine (F2). The correct ordering was obtained, however, by applying an extension of Koopmans’ theorem [J. Chem. Phys. 62, 113 (1975)] to an MC–SCF correlated wavefunction for F2 which contained only one configuration beyond the one used in Hartree–Fock. With the addition of still further configurations to the wavefunction for the neutral molecule, the ordering of the extended Koopmans’ valence orbital energies was maintained and the correspondence improved between those and the experimental values.