Accurate structures and binding energies for small water clusters: The water trimer

The global minimum on the water trimer potential energy surface has been investigated by means of second-order Moller-Plesset (MP2) perturbation theory employing the series of correlation-consistent basis sets aug-cc-pVXZ (X = D, T, Q, 5, 6), the largest of which contains 1329 basis functions. Definitive predictions are made for the binding energy and equilibrium structure, and improved values are presented for the harmonic vibrational frequencies. A value of 15.82+/-0.05 kcal mol(-1) is advanced for the infinite basis set frozen core MP2 binding energy, obtained by extrapolation of MP2 correlation energies computed at the aug-cc-pVQZ MP2 geometry. Inclusion of core correlation, using the aug-cc-pCV5Z basis set, has been found to increase the binding energy by 0.08 kcal mol(-1), and after consideration of core correlation and higher-order correlation effects, the classical binding energy for the water trimer is estimated to be 15.9+/-0.2 kcal mol(-1). A zero-point vibrational correction of -5.43 kcal mol(-1) has been computed from aug-cc-pVTZ MP2 harmonic vibrational frequencies. The accuracy of different computational schemes for obtaining the binding energies of the water dimer and trimer has been investigated, and computationally feasible methods are suggested for obtaining accurate structures and binding energies for larger water clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)30519-5].