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

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.