Symmetry adapted perturbation theory of intermolecular potentials. The helium–helium interaction

Abstract

The use of symmetry adapted perturbation theory to calculate intermolecular potentials in the important intermediate region is explored. A scheme is suggested which gives both the correct long range limit and the correct wavefunction symmetry order by order. A discussion of the accuracies of first order energies determined from approximate wavefunctions is given together with an approximate treatment of the second order energy. Finally, the overall potential obtained from this scheme for He₂ is shown to be in good agreement with recent semi-empirical and variationally determined potentials.