However, it is still often stated that the other orbital energies have no physical meaning, and that it is a pity that nothing like Koopmans' theorem to give meaning to the other occupied orbital energies exists. The truth could not be more different: it has been established22–25 that the KS orbital energies of the valence levels in molecules approximate the experimental ionization energies much more closely (typically at 0.1 eV level) than the Hartree–Fock orbital energies do (difference with IPs typically ∼1 eV).