The molecular-level relationship between the properties of liquid water molecules and orientational order

The relationship between the orientational (tetrahedral) order (q) of an individual liquid water molecule and its various properties such as Voronoi volume, potential energy, kinetic energy, and nearest neighbors was thoroughly examined using molecular dynamics simulations of TIP5P model at 278, 298, and 318 K. By constructing Voronoi polyhedra (VP), we found that the average volume of water molecules classified according to q decreased monotonically as q increased, while the surface of VP increased in the range of high q. Kinetic energy was almost invariant but potential energy decreased monotonously as q increased. The volumes of molecules having a very large q increased as temperature decreased, implying a possible density maximum phenomena. Using time correlation functions, it was shown that total energy rather than potential energy was a more significant factor in the determination of the orientational order. With varying temperature, the relation between the properties of central molecules and those of nearest neighbor molecules were investigated. It required a very systematic cooperative motion to obtain LDL (low-density liquid) formed by ordering. It was supposed that density maximum phenomena should be accomplished by the growth of LDL and HDL (high-density liquid) of low quality with a consistent population of HDL and a drop of LDL of high quality as temperature lowered.