The calculation of dispersion energies by the variation method

Variation perturbation calculations of the R -6 term in the long range expansion of the dispersion energy of two helium atoms are reported, based on S.C.F. and correlated wave functions of the helium atom. The various forms of the unperturbed hamiltonian are discussed. A simple first-order wave function built up from a Slater-type 2p orbital with optimized exponent on each helium atom is apparently sufficient to give quite accurate values for both the Hartree-Fock and correlated-atom dispersion energies.