Quantum Mechanics of the H2–H2 Interaction. III. Nonorthogonal SCF—GF Calculations in the One-Configuration Approximation

Abstract

Short‐range intermolecular forces in the H₂–H₂ system have been approached by a rigorous quantum‐mechanical calculation using a wavefunction consisting of one configuration of nonorthogonal group functions constructed from a minimal basis set of 1s Slater orbitals. The need of avoiding, with restricted wavefunctions, the forced orthogonalization of the atomic‐orbital basis, which amounts to the relaxation of the strong orthogonality condition in a group‐function approach, appears to be essential in order to give the correct angular dependence of the intermolecular energy. About 98% of the potential `barrier' resulting from the minimal‐basis‐set calculation with complete configuration interaction is accounted for by nonorthogonal group functions which allow only for intramolecular correlation. The even simpler one‐determinant wavefunction constructed from SCF bond orbitals gives numerical results within 5% to 10% of the reference value for the intersection and within 3% to 5% for the energy difference between different relative orientations. Both inter‐ and intramolecular correlations seem therefore to be relatively unimportant in determining the relative orientation of the two molecules in the short‐range region. The partitioning of the short‐range interaction energy into a Coulomb component (slightly attractive and slowly varying with the dihedral angle) and into a correction or penetration term, which arises from electron interchange between the electron groups when they begin to overlap, shows that the largest part of the interaction and its orientation dependence arise from the repulsion due to negative overlap between the closed‐shell charge clouds.