Quantum Mechanics of the H2–H2 Interaction. II. A Full Valence-Bond Calculation

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Abstract
The valence‐bond theory of molecular structure in its rigorous formulation is applied to investigate the orientation dependence of the short‐range interaction energy for two H2 molecules in their ground state. The electronic wavefunction is extended to include all the singlet valence‐bond (VB) structures arising from the minimal basis set of four 1s Slater orbitals; namely, two covalent, twelve singly polar, and six doubly polar structures having S=0 and MS=0. The occurring three‐ and four‐center molecular integrals have been computed by numerical integration to six significant figures. The resulting intermolecular energy and the electron distribution within the system are compared with those obtained in a preceding paper (Part I) using a wavefunction restricted to nine VB structures drawn from the same set of atomic orbitals after symmetrical orthogonalization. The present unrestricted calculation results not only in a much lower intermolecular energy than that obtained in I, but also in a substantially larger orientation dependence of the interaction. Since no orthogonality restriction is implied in the full valence‐bond wavefunction, the forced orthogonalization of the atomic‐orbital basis appears to be a rather severe constraint with electronic wavefunctions of a restricted form.

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