Correlation Effects in Atoms. II. Angular Correlations between Electrons

The variational-perturbation method is applied to the study of angular correlations between electrons in the helium atom. We compare the relative merits of various basis functions for calculating the correlation energies. Starting from the hydrogenic and Hartree-Fock Hamiltonians as zeroth-order approximations, we have performed detailed numerical calculations on the ground state of helium. Very accurate results are obtained with a set of basis functions intermediate between the Hylleraas and the "configuration-interaction" types of trial functions. The perturbation theory of the (1s2s) ^1S₀ and (1s2s) ^3S₁ states is also investigated.