Ground State of the Beryllium Atom:Reinvestigation Based on a Proper Independent Particle Model

摘要

As pointed out a long time ago by Hylleraas,Eckhart,Coulson,Shull,and Lowdin,the Hartree-Fock model does not provide the lower-energy wave function expressed as a single orbital product.Valence bond-like wave functions,with one configuration of optimized nonorthogonal singly occupied orbitals,provide the best possible independent particle model(IPM)wave function.In this article,we present IPM calculations,beyond the Hartree-Fock model,at the generalized valence bond and generalized multistructural(CMS)levels for the ground state of the beryllium atom.The electronic structure of the beryllium atom can be viewed as resonance hybrid of three configurations.A simple prescription to transform a CASSCF(2,4)to a GMS-3S wave function yielding a fully optimized and uniquely defined IPM is presented.With these wave functions,using a variety of basis sets it is shown that,contrary to what is usually assumed in the literature,the valence electronic correlation energy in the beryllium atom is small and the stabilization brought about by including the 1s~22p~2 configuration is neither due to nondynamic nor dynamic correlation effects.