Multi-Configuration Hartree–Fock Correlation Study of 1s2s1S

Abstract

A correlation study is performed for the 1s2s1S state of helium to neon. It is shown that the Hartree–Fock approximation is poor, and that the two-configuration approximation [Formula: see text] is equivalent to a nonorthogonal Hartree–Fock approximation for which Koopmans' theorem holds. Additional configurations are introduced which account for 90% of the correlation energy.