A theory for discriminating the mechanism responsible for the water density anomaly

We propose a model elucidating the mechanisms at the origin of the density anomaly of water. We start from a model of Sastry et al., with no correlation between the hydrogen bonds (HBs), that rationalizes the experiments with an anomalous behavior of the line of temperatures of maximum density (TMD), i.e., with a singularity-free scenario. We introduce the correlation of HBs and solve the model within a mean-field approach. The resulting phase diagram shows the same anomalous behavior of the TMD line but also the occurrence of a phase transition between a high- and a low-density liquids ending in a critical point. Therefore, the introduction of the HBs correlation is enough to pass from the singularity-free scenario to the second critical point scenario. Since experiments on water show the presence of HBs correlation, this result suggests that the relevant mechanism for the density anomaly is the liquid–liquid phase transition.