Z-1/2expansion of the unrestricted Hartree-Fock equations for the (1s)(1s')1S states of helium and helium-like ions: II

Abstract

It is shown that the unrestricted Hartree-Fock equations may be solved by expanding the orbital wave functions in perturbation series with parameter Z-1/2 The work of a previous paper is extended to the calculation of the orbital functions to second order, and the energy to fifth order A similar calculation is made for the restricted Hartree-Fock approximation The higher-order energy coefficients are estimated Using these estimates, the restricted Hartree-Fock energy sums to -2 861 68 au, which is the value obtained by Roothaan et al, while the unrestricted Hartree-Fock energy sums to -2 877 99 au, which is the value obtained by Froese.