Quantum Mechanics of the H2–H2 Interaction. IV. A Self-Consistent Group Calculation with Strong Orthogonal Group Functions

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Abstract
A self‐consistent group calculation of the orientation dependence of short‐range intermolecular forces in the H2–H2 system has been carried out using strong orthogonal Weinbaum‐like functions constructed from a set of four symmetrically orthogonalized 1s orbitals. The one‐configuration approximation to the wavefunction describing the electronic ground state of the composite system has been improved by admitting localized excitations on the two molecules. The resulting intermolecular energy is for some relative orientations slightly better and for others worse than that obtained in the corresponding nonorthogonal calculation, but the orientation dependence of the interaction proves to be definitely worst in the orthogonal case. Only about 60%of the ``barrier'' obtained with the one‐configuration nonorthogonal SCF—GF calculation is accounted for by the corresponding strong orthogonal wavefunction. The resolution of the one‐configuration interaction into Coulomb and penetration components after the distortion effects resulting from the forced orthogonalization of the orbital basis have been removed by means of a proper matrix technique shows that the largest error is embodied in the one‐electron contribution to the penetration term. The strong orthogonal SCF—GF—CI approach in its current formulation proves therefore not to be adequate in giving a satisfactory description of the orientation dependence of short‐range intermolecular forces, at least when the penetration interaction is the largest contribution.