Structure and dynamics of high- and low-density water molecules in the liquid and supercooled regimes

Abstract

By combining the local structure index with potential energy minimisations we study the local environment of the water molecules for a couple of water models, TIP5P-Ew and SPC/E, in order to characterise low- and high-density “species”. Both models show a similar behaviour within the supercooled regime, with two clearly distinguishable populations of unstructured and structured molecules, the fraction of the latter increasing with supercooling. Additionally, for TIP5P-Ew, we find that the structured component vanishes quickly at the normal liquid regime (above the melting temperature). Thus, while SPC/E provides a fraction of structured molecules similar to that found in X-ray experiments, we show that TIP5P-Ew underestimates such value. Moreover, unlike SPC/E, we demonstrate that TIP5P-Ew does not follow the linear dependence of the logarithm of the structured fraction with inverse temperature, as predicted by the two-order parameter model. Finally, we link structure to dynamics by showing that there exists a strong correlation between structural fluctuation and dynamics in the supercooled state with spatial correlations in both static and dynamic quantities.