Extrapolating the correlation energy

We present a general scheme for calculating the correlation energy in molecular systems by extrapolation of an energy Ž . expression, E t , obtained from a modified Hamiltonian whose range is reduced to induce localization of the physical Ž . interactions. The variable t is introduced such that one recovers the physical system for ts0 and E t s0 for tTM`. We show that very accurate correlation energies can be extrapolated at the MP2 level of theory using rational approximations to Ž . E t based on few evaluations of the energy function. Thus, if localization can be exploited to substantially reduce the computational cost and scaling of the energy evaluation, the scheme becomes a practical tool for large molecular applications. q 1999 Elsevier Science B.V. All rights reserved.