Bare-Nucleus and Screened-Nucleus Perturbation Theory for He2

Abstract

In the bare‐nucleus perturbation theory, where the full interelectronic repulsion is taken as the perturbing part of the Hamiltonian, determination of the first‐order wavefunction for a closed‐shell system reduces to a set of two‐electron problems. Only in the evaluation of the energy do three‐electron integrals appear. One can thus produce a correlated wavefunction (containing interelectronic distances explicitly) without evaluation of three‐electron integrals as arise in the variational method. After a review of the necessary formulas, we present calculations for He₂. The convergence of the energy series is disappointing. However, the same formalism can be used when any one‐electron local operator is taken as zero‐order Hamiltonian. In particular we present screened‐nucleus calculations, the zero‐order Hamiltonian corresponding to noninteracting electrons moving in the field of nuclei of charge 1.5. Here, our energy through second order is as good as the best variational calculations.