Accurate Atomic and Molecular Wave Functions Without Exchange

A perturbation-theoretic procedure is developed for obtaining the spatial function Φ₀ for the many-electron problem, from which the total wave function can be projected by the relation ${Ψ}={Σ}{i}^{}{{ ̃ \~{}{}}{D}}ᵢ₀(r){D}ᵢ₀({σ}){{Φ}}₀(r){{χ}}₀({σ})$. This function is expanded in a perturbation series in which the ${{{Φ}}₀}⁰$ contains a sufficient set of pair symmetries of ${{Φ}}₀$ itself, such as in the Hartree nonantisymmetrized wave function for closed-shell atoms. When the expansion converges, the remaining symmetries are introduced exactly. The energy eigenvalue does not contain the usual "exchange" terms, since the zeroth-order Hamiltonian, unlike the Hartree-Fock H₀, has no degeneracies. Applications to interaction energies in molecular crystals and asymmetric wave functions are discussed briefly.