Extrapolating the correlation energy

We present a general scheme for calculating the correlation energy in molecular systems by extrapolation of an energy expression, E(t), obtained from a modified Hamiltonian whose range is reduced to induce localization of the physical interactions. The variable t is introduced such that one recovers the physical system for t=0 and E(t)=0 for t→∞. We show that very accurate correlation energies can be extrapolated at the MP2 level of theory using rational approximations to 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.