Generalized Heitler-London theory for H3: a comparison of the surface integral method with perturbation theory

The generalized Heitler-London (GHL) theory provides a straightforward way to express the potential energy surface of H3 in terms of Coulomb and exchange energies which can be calculated either by perturbation theory or using the surface integral method (SIM). By applying the Rayleigh-Schrödinger perturbation theory, GHL theory for the quartet spin state of H3 is shown to yield results equivalent to the symmetrized Rayleigh-Schrödinger version of symmetry adapted perturbation theory (SAPT). This equivalence allows a comparison with the corresponding results obtained by the surface integral method. The surface integral result calculated with a product of atomic wave functions is found to have certain advantages over the perturbation approach