Two-particle entropy and structural ordering in liquid water

Entropies of simple point charge (SPC) water were calculated over the temperature range 278-363 K using the two-particle correlation function approximation. Then, the total two-particle contribution to the entropy of the system was divided into three parts, which we call translational, configurational, and orientational. The configurational term describes the contribution to entropy, which originates from spatial distribution of surrounding water molecules (treated as points, represented by the center of mass) around the central one. It has been shown that this term can serve as the metric of the overall orientational ordering in liquid water. Analyzing each of these three terms as a function of intermolecular distance, r, we also find a rational definition of the hydration shell around the water molecule; the estimated radii of the first and second hydration shells are 0.35 nm and 0.58 nm, respectively. We find, moreover, that the first hydration shell around the water molecule participates roughly in 70% of the total orientational entropy of water, and this rate is roughly temperature independent.