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