Characterization of the Local Structure in Liquid Water by Various Order Parameters

A wide range of geometric order parameters have been suggested to characterize the local structure of liquid water and its tetrahedral arrangement, but their respective merits have remained elusive. Here, we consider a series of popular order parameters and analyze molecular dynamics simulations of water, in the bulk and in the hydration shell of a hydrophobic solute, at 298 and 260 K. We show that these parameters are weakly correlated and probe different distortions, for example the angular versus radial disorders. We first combine these complementary descriptions to analyze the structural rearrangements leading to the density maximum in liquid water. Our results reveal no sign of a heterogeneous mixture and show that the density maximum arises from the depletion in interstitial water molecules upon cooling. In the hydration shell of the hydrophobic moiety of propanol, the order parameters suggest that the water local structure is similar to that in the bulk, with only a very weak depletion in ordered configurations, thus confirming the absence of any iceberg-type structure. Finally, we show that the main structural fluctuations that affect water reorientation dynamics in the bulk are angular distortions, which we explain by the jump hydrogen-bond exchange mechanism.