STRUCTURES AND ENERGETICS OF F-(H2O)(N), N=1-3, CLUSTERS FROM AB-INITIO CALCULATIONS

We have computed the optimal structures and harmonic vibrational frequencies of the F-(H2O)(n), n = 1-3, clusters at the MP2 level of theory. For the n = 2 and 3 cases, the minimum-energy configurations correspond to asymmetric structures where the ion sits outside the water cluster. The electronic energy difference between these configurations and the symmetric ones, in which the ion is completely surrounded by water molecules is, however, very small (0.5-1.5 kcal/mol). When zero-point vibrational energy corrections are included, the energy difference becomes negligible and, in the n = 3 case, it is reversed making the symmetric structure favorable by 0.1 kcal/mol. It is therefore concluded that the dynamical effects in addition to the static features of the potential energy surface are important in determining the most probable orientation of the water molecules around the ion. The energy differences between the critical points were analyzed in terms of the relative magnitudes of the second- and higher-order interaction energy terms. We have found that electron correlation is very important in the computation of accurate energy differences and many-body interaction energy terms, especially for the n = 2 and 3 cases.