The additivity of second-order contributions to intermolecular potentials: calculations on the3Σu+state of H2

Variational calculations have been carried out on the 3Σ u + state of H2 in the region of the van der Waals minimum using a basis of states that introduce dispersion, induction and charge-transfer energies. The basis is constructed from orbitals with optimum exponents and leads to a total energy which is close to the best obtained from James-Coolidge type calculations. From this a meaningful subdivision of the total energy into dispersion, induction, second-order exchange and charge-transfer energies can be made, and the additivity of these energies examined. The sum of the dispersion and induction energies (calculated without exchange) and the charge-transfer energy, is close to the total stabilization energy deduced from the full variational calculation.