Structural features of high-local-density water molecules: Insights from structure indicators based on the translational order between the first two molecular shells.

The two-liquids scenario for liquid water assumes the existence of two competing preferential local molecular structural states characterized by either low or high local density. While the former is expected to present good local order thus involving privileged structures, the latter is usually regarded as conforming a high-entropy unstructured state. A main difference in the local arrangement of such "classes" of water molecules can be inferred from the degree of translational order between the first and second molecular shells. This is so, since the low-local-density molecules present a clear gap between the first two shells while in the case of the high-local-density ones, one or more molecules from the second shell have collapsed toward the first one, thus populating the intershell region. Some structural indicators, like the widely employed local structure index and the recently introduced ζ index, have been devised precisely on the basis of this observation, being successful in detecting well-structured low-local-density molecules. However, the nature of the high-local-density state has been mainly disregarded over the years. In this work we employ molecular dynamics simulations for two water models (the extended simple point charge model and the five-site model) at the liquid and supercooled regimes combined with the inherent dynamics approach (energy minimizations of the instantaneous configurations) in order to both rationalize the detailed structural and topological information that these indicators provide and to advance in our understanding of the high-density state.