MODEL STUDIES OF 6-MEMBERED WATER CLUSTERS

Using the molecular mechanics for clusters model, calculations have been performed that explore the types of minimum energy structures that occur for a collection of six water molecules. The results show an extensive set of structures. The global minimum structure is best described as a stack of two three-membered rings with nine close (around 3 angstrom) molecule-molecule separations, or "bonds". Energetically less favorable structures occur mostly with fewer of these weak bonds; however, the energy per bond is greater in certain of the structures. In spite of the considerable variation in structures, there is a high degree of correlation between the average molecule separation distances and the stabilities for all the equilibrium structures. Furthermore, several calculations on clusters of other than six waters show the same correlation. Harmonic vibrational frequencies are presented for the most stable structures, and comparison calculations with two other widely used water potentials are reported.