A PERTURBATION CALCULATION OF THE GROUND-STATE (X 1-SIGMA-G+) ENERGY OF THE HYDROGEN MOLECULE

The generalized Heitler-london theory has been implemented for a calcu lation of the ground state (X 1SIGMA(G)+) potential of the H-2 molecul e at distances of R=1.4 and 8.0 au. With the exact exchange energy and newly calculated accurate overlap integrals, convergence is achieved to better than 1% by expanding only up to the tenth order. With the ex change energy calculated in zeroth-order by the surface integral metho d, the agreement is quite good at most distances and can be improved t o better than 6% by extending the perturbation series to 60th order.