ACCURATE HEITLER-LONDON INTERACTION ENERGY FOR HE-2

A 75-term Gaussian geminal wave function for the helium atom that has a variational energy within 0.42 mu hartree of the exact one is constr ucted. It predicts an electron density that agrees to better than 0.4% with the predictions of energetically superior Hylleraas wave functio ns to electron-nucleus distances as large as 6a(0). The first-order He itler-London interaction energy E((1)) between a pair of helium atoms was computed using an antisymmetrized product of this Gaussian geminal wave function for each of the atoms. This interaction energy is an es sential component in the exchange-Coulomb model for the He-2 potential . Our E((1)) is probably converged to within 0.03 mu hartree for inter atomic distances between 3.0a(0) and 7.5a(0) The Coulombic part of the interaction energy was checked by computations using even more accura te Hylleraas wave functions for the monomers Comparison with an E((1)) value computed from self-consistent-field atomic wave functions shows that intra-atomic correlation effects range between 4% and 9%.