We introduce the space-dependent correlation function C Q (r) and time-dependent autocorrelation function C Q (t) of the local tetrahedral order parameter Q ≡ Q(r, t). By using computer simulations of 512 waterlike particles interacting through the transferable interaction potential with five points (TIP5 potential), we investigate C Q (r) in a broad region of the phase diagram. We find that at low temperatures C Q (t) exhibits a two-step time-dependent decay similar to the self-intermediate scattering function and that the corresponding correlation time τ Q displays a dynamic cross-over from non-Arrhenius behavior for T > T W to Arrhenius behavior for T < T W , where T W denotes the Widom temperature where the correlation length has a maximum as T is decreased along a constant-pressure path. We define a tetrahedral entropy S Q associated with the local tetrahedral order of water molecules and find that it produces a major contribution to the specific heat maximum at the Widom line. Finally, we show that τ Q can be extracted from S Q by using an analog of the Adam-Gibbs relation.specific heat of water | orientational entropy of tetrahedral liquids | anomalies of liquid water | Widom line I t has long been appreciated that the local structure around a molecule of liquid water arising from the vertices formed by four nearest neighbors is approximately tetrahedral at ambient pressure and that the degree of tetrahedrality increases when water is cooled (1-5). An important advance occurred in the past 10 years when computer simulations allowed the quantification of the degree of tetrahedrality (6-10) by assigning to each molecule a local tetrahedral order parameter Q (11-16). At high temperatures, the probability distribution P(Q, T) is bimodal, with one peak corresponding to a high degree of tetrahedrality and the other to a less tetrahedral environment (Fig. 1). Upon decreasing temperature, the peak associated with a high degree of tetrahedrality grows, suggesting that the local structure of water becomes much more tetrahedral at lower temperatures (13,14,17).
IntroductionWater has been hypothesized to belong to the class of polymorphic liquids, phase separating-at sufficiently low temperatures and high pressures-into two distinct liquid phases: a high density liquid (HDL) with smaller Q and a low density liquid (LDL) with larger Q (18). The coexistence line separating these two phases may terminate at a liquid-liquid (LL) critical point, above which (in the LL supercritical region) appears a line of correlation length maximum in the pressure-temperature plane. The locus of maximum correlation length in the one-phase region is called the Widom line T W ≡ T W (P) (19), near which different response functions, such as isobaric specific heat C P and isothermal compressibility K T , display maxima. Recent neutron-scattering experiments (20) and computer simulations (19) show that the dynamics of water gradually cross over from being non-Arrhenius for T > T W to Arrhenius for T < T W .Although dynamic heterogeneities h...