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

We have computed the optimal structures and harmonic vibrational frequ encies 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 corresp ond to asymmetric structures where the ion sits outside the water clus ter. The electronic energy difference between these configurations and the symmetric ones, in which the ion is completely surrounded by wate r molecules is, however, very small (0.5-1.5 kcal/mol). When zero-poin t vibrational energy corrections are included, the energy difference b ecomes negligible and, in the n = 3 case, it is reversed making the sy mmetric 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 probabl e orientation of the water molecules around the ion. The energy differ ences between the critical points were analyzed in terms of the relati ve magnitudes of the second- and higher-order interaction energy terms . We have found that electron correlation is very important in the com putation of accurate energy differences and many-body interaction ener gy terms, especially for the n = 2 and 3 cases.