Koopmans' theorem

Directly relates experimental ionization potentials with energy levels of molecular orbitals. The theorem states that the ionization potential required to remove an electron from the orbital Ψ i is given by the negative value of the energy of the orbital, − ε i, as calculated within the Hartree–Fock approximation. The theorem is not applied to localized molecular orbitals, which are not eigenfunctions of the effective hamiltonian.
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1949