Optimized partitioning in perturbation theory: Comparison to related approaches

A generalized Epstein-Nesbet type perturbation theory is introduced by a un ique, "optimal" determination of level shift parameters. As a result, a new partitioning emerges in which third order energies are identically zero, m ost fifth order terms also vanish, and low (2nd, 4th) order corrections are quite accurate. Moreover, the results are invariant to unitary transformat ions within the zero order excited states. Applying the new partitioning to many-body perturbation theory, the perturbed energies exhibit appealing fe atures: (i) they become orbital invariant if all level shifts are optimized in an excitation subspace; and (ii) meet the size-consistency requirement if no artificial truncations in the excitation space is used. As to the num erical results, low order corrections do better than those of Moller-Plesse t partitioning. At the second order, if the single determinantal Hartree-Fo ck reference state is used, the CEPA-0 (=LCCD) energies are recovered. High er order corrections provide a systematic way of improving this scheme, num erical studies showing favorable convergence properties. The theory is test ed on the anharmonic linear oscillator and on the electron correlation ener gies of some selected small molecules. (C) 2000 American Institute of Physi cs. [S0021-9606(00)31210-7].