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Herbert H. H.Homeier
Institut für Physikalische und Theoretische
Chemie
Universität Regensburg
D-93040
Regensburg,
Germany
Estimators 
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 
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 for n>2 are size-extensive
if they are calculated from the MP series.
Keywords: Convergence acceleration -- ab initio method -- Extrapolation -- Many-body perturbation theory -- Møller-Plesset series