Hellmann-Feynman Theorem and Correlation Energies

A proof is offered for the applicability of the Hellmann-Feynman theorem to Hartree-Fock wave functions. In conjunction with Sinanoğlu's theory of correlation energies, this leads to the idea that Hartree-Fock potential energy surfaces and exact potential energy surfaces are parallel over short distances. Finally, a new method of calculating dissociation energies is presented.