Basis-set convergence of correlated calculations on water

Abstract

The basis-set convergence of the electronic correlation energy in the water molecule is investigated at the second-order Mo/ller–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-consistentbasis 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.