Koopmans' theorem

https://doi.org/10.1351/goldbook.K03411
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 Ψi is given by the negative value of the energy of the orbital, εi, as calculated within the
Hartree
–Fock approximation. The theorem is not applied to localized molecular orbitals, which are not eigenfunctions of the effective hamiltonian.
Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1949 [Terms] [Paper]