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 H_3 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-Schroedinger perturbation theory, GHL theory for the quartet spin state of H_3 is shown to yield results equivalent to the symmetrized Rayleigh-Schroedinger 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.