The size-extensitivity of correlation energy estimators based on effective characteristic polynomials

Abstract

Estimators $\Pi n$ for the correlation energy can be computed as roots of effective characteristic polynomials of degree $n$. The coefficients of these polynomials are derived from the terms of the perturbation series of the energy. From a fourth-order Møller-Plesset (MP4) calculation one can calculate with negligible effort a size-extensive estimator $\Pi 2$ that is in many cases much closer to the full CI correlation energy of the ground state than the MP4 value. [H.H.H. Homeier, J. Mol. Struct. (Theochem) 366, 161 (1996)] Here, we prove that the estimators $\Pi n$ for $n>2$ are size-extensive if they are calculated from the MP series.

6 pages, 10 references, LaTeX2e, elsart.cls, J. Mol. Struct. (THEOCHEM), in press. Paper 27 at the 3${}^{rd}$ Electronic Computational Chemistry Conference, 1996. Postscript and dvi also available via http://www.chemie.uni-regensburg.de/preprint.html