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.