Modulated perturbation theory for molecular interactions. Part 1.—An exact second-order calculation for the ground state of H +2

Abstract

A perturbation theory which includes exchange appropriate for calculations of molecular interactions over the whole range of internuclear separations from the united atom to the separate atoms is proposed in terms of a modified MS–MA approach. Modulation of H₀ by suitable parameters is introduced to a first order to keep the electronic component of the Coulombic perturbation V small at all separations, so giving better convergence of the perturbation expansion. An exact calculation of the interaction energy for the ²Σ⁺_g state of H⁺₂ shows that the results obtained in second order are capable of accuracy over the whole range of internuclear distances. The separation of the interaction energy into Coulombic and penetration components allows a detailed analysis of the physical nature of the interaction to be made.