Low- and high-density forms of liquid water revealed by a new medium-range order descriptor

We present in this paper a computational approach based on molecular dynamics simulations and graph theory to characterize the structure of liquid water considering not only the local structural arrangement within the first (or second) hydration shell, but also the medium- to long-range order. In particular, a new order parameter borrowed from the graph-theory framework, i.e., the node total communicability (NTC), is introduced to analyze the dynamic network of water molecules in the liquid phase. This order parameter is able not only to accurately report on the different high-density-liquid (HDL) and low-density-liquid (LDL) water phases postulated in the liquid-liquid phase transition hypothesis, but also to show that HDL-like forms are not homogeneous but rather composed by regions at different local density. In particular, the presence of patches at very high density with an increased number of interstitial water molecules is unveiled, both under pressure and at ambient conditions.