Accurate Heitler-London interaction energy for He2

A 75-term Gaussian geminal wave function for the helium atom that has a variational energy within 0.42 μhartree of the exact one is constructed. It predicts an electron density that agrees to better than 0.4% with the predictions of energetically superior Hylleraas wave functions to electron-nucleus distances as large as 6a₀. The first-order Heitler-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 essential component in the exchange-Coulomb model for the He₂ potential. Our E⁽¹⁾ is probably converged to within 0.03 μhartree for interatomic distances between 3.0a₀ and 7.5a₀. The Coulombic part of the interaction energy was checked by computations using even more accurate 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%.