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 leve l and at the coupled-cluster singles-and-doubles level with and withou t perturbative triples corrections applied. The basis-set limits of th e correlation energy are established to within 2 mE(h) by means of (1) extrapolations from sequences of calculations using correlation-consi stent basis sets and (2) from explicitly correlated calculations emplo ying terms linear in the interelectronic distances r(ij). For the extr apolations 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 tri ple-zeta sets, etc.) are found to be useful. CCSD(T) calculations invo lving as many as 492 atomic orbitals are reported. (C) 1997 American I nstitute of Physics.