Basis-set convergence of correlated calculations on water

The basis-set convergence of,the electronic correlation energy in the water molecule is investigated at the second-order Moller-Plesset level and at the coupled-cluster singles-and-doubles level with and without perturbative triples corrections applied. The basis-set limits of the correlation energy are established to within 2 mE(h) by means of (1) extrapolations from sequences of calculations using correlation-consistent basis sets and (2) from explicitly correlated calculations employing terms linear in the interelectronic distances r(ij). For the extrapolations to the basis-set limit of the correlation energies, fits of the form a+bX(-3) (where X is two for double-zeta sets, three for triple-zeta sets, etc.) are found to be useful. CCSD(T) calculations involving as many as 492 atomic orbitals are reported. (C) 1997 American Institute of Physics.