Computer simulation of bilayer ice: structures and thermodynamics

A series of molecular dynamics simulations is performed in order to examine in more detail the results of a previous simulation which shows that a thin film of water, when confined to a hydrophobic slit nanopore, freezes into a bilayer ice crystal composed of two layers of hexagonal rings. Three simulations are carried out and each starts with a different initial configuration but has the same number of molecules and the area density. Using a previously introduced solid-like cluster definition, we monitor the dynamic process of crystallization. We find that only in one case the confined water completely freezes into perfect bilayer of ice whereas in other two cases, an imperfect crystalline structure consisting of hexagons of slightly different shapes is observed and this imperfection apparently hinders the growth of perfect bilayer of crystal. After adjusting the area density to match spatial arrangements of molecules, the latter two systems are able to crystallize completely. As a result, we obtain three forms of bilayer crystal differing in the area density and hexagonal rings alignment. Further analyses of these bilayer crystals provide more insightful explanation on the influence of the boundary condition and the simulation-cell size on the diversity of possible crystallographic structures.