(563d) Thermodynamically Sensitive Coordination Structures of Liquid Water

Within a rigorous theoretical framework the hydration free energy of a water molecule is separated into: a chemical term arising due to interactions of a distinguished water molecule with water molecules within a defined coordination shell, a packing contribution accounting for the formation of an empty cavity and a long range piece detailing the interactions of the distinguished water molecule with water molecules outside the coordination sphere. It is observed that as the size of the coordination sphere is increased, the long range piece diminishes in magnitude and begins to be increasingly well described by gaussian models of hydration. Indeed, for very large coordination spheres the hydration free energy is almost entirely given by the sum of the chemical and packing terms. The packing term is further recast as a sum over various n-coordinate states with each step wise increment in the coordination number more fully accounting for the chemical contribution. Such a molecular aufbau approach can be used to interrogate the thermodynamic importance of various coordinate states. For the SPC/E model of water it is observed that though the closest four water molecules contribute significantly to the chemical term in reasonably sized coordination spheres, it is only the closest water that shows a sharp drop in its chemical contribution as the size of the coordination sphere is increased. Thus for the SPC/E model, the interaction of only the closest water can be thought to be chemically specific.