Basis set convergence of correlated calculations on HF, H 2O, N 2, CO, and Ne

Abstract

The convergence behavior of correlation and total (Hartree–Fock+correlation energy) electronic energies of HF, H 2O, N 2, CO, and Ne at the MP2 and CCSD( T) levels with correlation consistent basis sets aug-cc-pV XZ and aug-cc-pCV XZ ( X= D(2), T(3), Q(4),5,6) was examined by employing an extrapolation scheme which fits two successive correlation consistent basis set energies by ( X+ k) −3 and ( X+ k) − p ( k=0 at the MP2 and k=−1 at the CCSD( T) level) to estimate the basis set limit. The convergence power p in the ( X+ k) − p expansion was determined by averaging the optimum exponents which can reproduce the known basis set limits of these systems through extrapolation of two successive energies with XZ and ( X+1)Z bases, and the resulting values for p and the suitability of correlation or total energies for the extrapolation to estimate the corresponding basis set limits are shown to depend on electron correlation level and basis sets employed. While the correlation energies than total energies are better suited for extrapolation to estimate the basis set limit at the MP2 level with large basis sets, the extrapolation of total energies rather than correlation energies appears in general more appropriate to estimate the basis set limit in both ( X+ k) −3 and ( X+ k) − p extrapolations. The MP2 energies were always found to converge more slowly with basis set than CCSD( T) energies.