Basis-set extrapolation

A proposal for extrapolation of correlated electronic structure calculations based on correlation-consistent polarized double- and triple-zeta basis sets is evaluated. Optimum exponents are presented for separately extrapolating the Hartree-Fock and correlation energies, and the method is shown to yield energies that are more accurate than those from straight correlation-consistent polarized sextuple-zeta calculations at less than 1% of the cost. For the test problems, the root-mean-square deviations from the complete basis limit are 1.3-2.4 kcal/mol for the extrapolated calculations and 3.0-4.4 kcal/mol for the polarized sextuple-zeta calculations.