Energetics of Water Polyhedra with Square Faces.

In ice and other icelike systems, the structural variety of tetrahedrally coordinated systems increases strongly due to proton disorder. Configurations that differ from each other only by the arrangement of hydrogen atoms (protons) in hydrogen bonds are nonequivalent. Polyhedral water clusters are interesting objects of study since the analysis of the energetics of these tetrahedrally coordinated systems reveals very simple regularities. For water polyhedra of the most regular shape, the simplified physical model of strong and weak effective H bonds (the SWEB model) has been developed. This model allows predicting the classes of the most stable proton configurations. The number of H bonds that are more preferable from the point of view of Coulomb interaction is the only classification criterion for this model. For water polyhedra with square faces, this model becomes less accurate. However, for such clusters, there is another essential topological indicator: the number of molecules that are completely located in the planes of square faces. The results of numerous calculations show that these two topological indices largely determine the energetics of a huge number of proton configurations in water polyhedra with square faces.