The structural validity of various thermodynamical models of supercooled water

The thermodynamic response functions of water exhibit an anomalous increase upon cooling that becomes strongly amplified in the deeply supercooled regime due to structural fluctuations between disordered and tetrahedral local structures. Here, we compare structural data from recent x-ray laser scattering measurements of water at 1 bar and temperatures down to 227 K with structural properties computed for several different water models using molecular dynamics simulations. Based on this comparison, we critically evaluate four different thermodynamic scenarios that have been invoked to explain the unusual behavior of water. The critical point-free model predicts small variations in the tetrahedrality with decreasing temperature, followed by a stepwise change at the liquid-liquid transition around 228 K at ambient pressure. This scenario is not consistent with the experimental data that instead show a smooth and accelerated variation in structure from 320 to 227 K. Both the singularity-free model and ice coarsening hypothesis give trends that indirectly indicate an increase in tetrahedral structure with temperature that is too weak to be consistent with experiment. A model that includes an apparent divergent point (ADP) at high positive pressure, however, predicts structural development consistent with our experimental measurements. The terminology ADP, instead of the commonly used liquid-liquid critical point, is more general in that it focuses on the growing fluctuations, whether or not they result in true criticality. Extrapolating this model beyond the experimental data, we estimate that an ADP in real water may lie around 1500 ± 250 bars and 190 ± 6 K.