Ionization energies of open-shell molecules in the frozen orbital approximation

Abstract

We present a generalization of Koopmans' theorem applicable to molecules with any number of open shells, and with orbital degeneracy. For each shell, an average ionization energy is defined by weighting ionized states by their intensity in the photoelectron spectrum. In the frozen-orbital approximation, this average is the negative of the orbital energy obtained from a separate-hamiltonian restricted Hartree-Fock calculation on the molecular ground state. The result is less rigorous than for closed-shell molecules, since no choice of molecular orbitals will completely eliminate hamiltonian matrix elements between states of different ionized configurations.