Optimized partitioning in Rayleigh-Schrodinger perturbation theory

Finite-order perturbation corrections are ambiguous since they depend on th e partitioning of the Hamiltonian to a zero-order part and perturbation, an d any chosen partitioning can be freely modified, e.g, by level shift proje ctors. To optimize low-order corrections, an approximate variational proced ure is proposed to determine level shift parameters from the first-order An satz for the wavefunction. The resulting new partitioning scheme provides s ignificantly better second-order results than those obtained by standard pa rtitions like Epstein-Nesbet or Moller-Plesset. We treat the anharmonic osc illator and the atomic electron correlation energy in He, Be and Ne as nume rical test cases. (C) 1999 Published by Elsevier Science B.V. All rights re served.