Reduced Partitioning Procedure in Configuration Interaction Studies. I. Ground States

Abstract

A variation-perturbation approach to configuration interaction (CI) studies of atoms and molecules has been derived which yields a series of converging upper bounds to the CI energy. As a consequence, the method permits an adequate representation of a large n-particle wavefunction in terms of a very low order nonlinear perturbation summation. This ``reduction'' procedure is found to have simplifications for full CI studies as well as suggesting certain systematic improvements over the standard Hartree-Fock (HF) and ``truncated'' CI calculations. The first order solution, which is equivalent to the application of a geometric sumrule to the perturbation expansion is also shown to be derivable from a ``steepest descent'' argument. In preliminary calculations on H2 and the HeH+ molecular ion with a HF function as the unperturbed state, the first order result gives 60% to 70% of the energy difference between the full CI energy and the HF value, while only a third order treatment accounts for better than 90%. Potential curves for the ground state of HeH+ for the eight lowest orders of solution and their spectroscopic parameters are also obtained, with the eighth order energies differing from the full CI values by less than 10−4 a.u. at all internuclear separations.