Variational calculations of the static polarizability of H2and of the dipole-dipole contribution to the dispersion energy between two hydrogen molecules

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Abstract
The convergence of variation-perturbation calculations using non-linear variation parameters has been examined for the static polarizability of H2 and for the coefficient (C6) of the R -6 term in the multipole expansion of the H2-H2 dispersion energy. Using a correlated ground state wavefunction, and one excited state for each of the parallel and perpendicular components of the polarizability, a single variational parameter for each state gave 96 per cent of the exact value of α⊥ and 91 per cent of the exact value of α‖. A second variational parameter for the parallel component gave 94 per cent of α‖. The same procedure gave the spherical average and anisotropy of C6 to an accuracy of about 90 per cent. By extending the calculations into the region where overlap of the orbitals of the two molecules becomes significant, it is shown for the end-on configuration that the C6 R -6 term underestimates the dispersion energy near the van der Waals' minimum by roughly 10 per cent.

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