Quantitative characteristics of structure inhomogeneity of water

Abstract

Inhomogeneity of the local surroundings of molecules was studied for water models obtained by the molecular dynamic method. For all 3456 molecules in an independent unit cell, the volume of the Voronoi polyhedron (VVP) constructed around the oxygen atom, the tetrahedricity index (T), and the potential energy of the molecule (E _pot) were calculated. Based on the values of these parameters, the molecules were grouped into two classes. The first class included molecules for which this parameter was smaller than a certain critical value; the second included molecules for which the parameter exceeded the critical value. The critical value was chosen (for each configuration) such that each class contained exactly 50% of all molecules. Then it was investigated how many molecules belonged to each of these classes among the four nearest neighbors of the given sort. Deviation from the value obtained for random distribution of molecules of two sorts can serve as a measure of structure inhomogeneity, which shows itself as the tendency of a molecule to be surrounded by alike molecules. Since approximately half of all molecules are donors (and acceptors) for two hydrogen bonds at room temperature (a ₂ d ₂ type of coordination), a similar procedure is applicable to types of coordination of molecules. Molecules with a ₂ d ₂ coordination can be assigned to one class, while all other molecules can be considered as belonging to the other. Inhomogeneity of structure is most conspicuous for distribution of molecules with low and high values of the VVP (this is especially true of the surroundings of molecules with small values of VVP). The environment of molecules with high values of E _pot and of molecules with surroundings other than a ₂ d ₂ is nearly random.