Modified Perturbation Theory for Atoms and Molecules Based on a Hartree–Fock φ

The improvement of Hartree–Fock wavefunctions by perturbation theory is hampered by slow convergence with the usual choice of H0 = ∑ i = 1N F(i), where F denotes the one-electron Hartree–Fock Hamiltonian. The slow convergence is usually attributed to the unphysical nature of the excited states of F, which do not describe electrons moving in the field of the nuclei, shielded by N-1 electrons. It is shown how to redefine F so that the zeroth-order wavefunction still remains the Hartree–Fock wavefunction but so that the excited states of F correspond to an electron moving in the field of the nuclei screened by N-1 electrons. The redefined F thus results in a more appropriate H0. Calculations of some second-order polarization and semi-internal correlation energies in first-row atoms are given to illustrate the use of the redefined F.