Geometries, energies, and electrostatic properties of nonadditively optimized small water clusters

Several local energy minima have been determined for small water clusters, (H2O)n, of n=3, 4, and 6 molecules. Their geometries were optimized with the nonadditive potential of Campbell and Mezei. For n=6, the inclusion of the nonadditive component of the potential altered the order of the energies of the local minima on the energy surface. Whereas the additive approximation (pairwise energy sum) favored a nearly planar ring, the nonadditive energy preferred a staggered hexagon, as found in ice Ih. It has been shown that the nonadditive component—and even the energy contribution from the cooperative reinforcement of the induced dipole fields—are larger than the energy differences between different ice forms with their very different orientations. When both nonadditive and dispersion energy contributions were included, the equilibrium oxygen–oxygen distance for (H2O)6 was reduced from the optimal dimer distance to the range of vibrationally averaged oxygen–oxygen distances in condensed phases. Electric fields and substantially enhanced molecular dipole vectors have been calculated. The molecular dipole moments in the clusters are substantially larger than the isolated molecule moment.