Quantum field theoretical methods in chemically bonded systems IV

Summary

A new perturbative procedure is analyzed numerically for four single bonded diatomic molecules. The starting model is the second-quantized self-consistent Heitler-London model. The unperturbed function is a four-determinant Bardeen-Cooper-Schrieffer function. The model Hamiltonian is the ordinary Hamiltonian plus linear and quadratic powers of a two-level number operator. Parameters which multiply the additional terms are chosen to enforce particle-number symmetry. Convergence of the perturbative series for the energy as a function of internuclear distance is reasonable: third-order corrections are about an order of magnitude smaller than second-order corrections; total corrections through third order are about two orders of magnitude smaller than first-order energies.