Development of transferable interaction models for water. II. Accurate energetics of the first few water clusters from first principles

We report accurate energetics for the water trimer through pentamer global ring minima and four low-lying hexamer isomers (cage, prism, book, cyclic S6) from first principles electronic structure calculations. The family of augmented correlation-consistent orbital basis sets of double through quintuple zeta quality was used in order to estimate complete basis set (CBS) limits for the cluster total association energies at the second order perturbation (MP2) level of theory. These are −15.8 kcal/mol (trimer), −27.6 kcal/mol (tetramer), −36.3 kcal/mol (pentamer), −45.9 kcal/mol (prism hexamer), −45.8 kcal/mol (cage hexamer), −45.6 kcal/mol (book hexamer), and −44.8 kcal/mol (ring S6 hexamer). Effects of higher correlation, estimated at the coupled cluster plus single and double with a perturbative estimate of the triple excitations [CCSD(T)] level of theory, as well as inclusion of estimates for core–valence correlation suggest that these estimates are accurate to within 0.2 kcal/mol.