BASIS-SET CONVERGENCE IN CORRELATED CALCULATIONS ON NE, N-2, AND H2O

Valence and all-electron correlation energies of Ne, N-2, and H2O at f ixed experimental geometries are computed at the levels of second-orde r perturbation theory (MP2) and coupled cluster theory with singles an d doubles excitations (CCSD), and singles and doubles excitations with a perturbative triples correction (CCSD(T)). Correlation-consistent p olarized valence and core-valence basis sets up to sextuple zeta quali ty are employed. Guided by basis-set limits established by r(ij)-depen dent methods, a number of extrapolation schemes for use with the corre lation-consistent basis sets are investigated. Among the schemes consi dered here, a linear least-squares procedure applied to the quintuple and sextuple zeta results yields the most accurate extrapolations. (C) 1998 Elsevier Science B.V.