Efficient calculation of canonical MP2 energies

An efficient canonical second-order Møller–Plesset theory (MP2) procedure, based on the Saebo–Almlöf integral–direct transformation technique, coupled with efficient prescreening of the atomic orbital (AO) integrals, is described. For large molecules, a fraction of the AO integrals suffices to produce energies to microhartree accuracy. Calculations up to 1800 basis functions and 240 correlated electrons have been performed on a single processor computer for symmetrical molecules. Calculations with ∼1000 basis functions and ∼120 electrons can be performed routinely for molecules with no symmetry. However, scaling of the second half transformation is still steep. Several basis sets used in correlated calculations are compared for economy and performance.