Hydrogen-bond dynamics for the extended simple point-charge model of water

We study hydrogen-bond dynamics in liquid water at low temperatures using molecular dynamics simulations. We analyze the dynamics using energetic and geometric definitions of a hydrogen bond, and employ two analysis methods: (i) a history-dependent correlation function, related to the distribution of bond lifetimes, and (ii) a history-independent correlation function. For method (i) we find an approximately Arrhenius temperature dependence of the bond lifetime, and find that the distribution of bond lifetimes is extremely sensitive to the choice of bond definition. For method (ii) we find-independent of bond definition-that the dynamics are consistent with the predictions of the mode-coupling theory, suggesting that the slow dynamics of hydrogen bonds can be explained in the same framework as standard transport quantities. Our results allow us to clarify the significance of the choice of both bond definition and analysis technique.

Instantaneous normal mode analysis of supercooled water

We use the instantaneous normal mode approach to provide a description of the local curvature of the potential energy surface of a model for water. We focus on the region of the phase diagram in which the dynamics may be described by mode-coupling theory. We find that the diffusion constant depends on the fraction of directions in configuration space connecting different local minima, supporting the hypothesis that the dynamics are controlled by the geometric properties of configuration space. Furthermore, we find a relation between the number of basins accessed in equilibrium and the connectivity between them.

Configurational entropy and diffusivity of supercooled water

As a liquid approaches the glass transition, its properties are dominated by local potential minima in its energy landscape. The liquid experiences localized vibrations in the basins of attraction surrounding the minima, and rearranges via relatively infrequent inter-basin jumps. As a result, the liquid dynamics at low temperature are related to the system's exploration of its own configuration space. The 'thermodynamic approach' to the glass transition considers the reduction in configuration space explored as the system cools, and predicts that the configurational entropy (a measure of the number of local potential energy minima sampled by the liquid) is related to the diffusion constant. Here we report a stringent test of the thermodynamic approach for liquid water (a convenient system to study because of an anomalous pressure dependence in the diffusion constant). We calculate the configurational entropy at points spanning a large region of the temperature-density plane, using a model that reproduces the dynamical anomalies of liquid water. We find that the thermodynamic approach can be used to understand the characteristic dynamic anomalies, and that the diffusive dynamics are governed by the configurational entropy. Our results indicate that the thermodynamic approach might be extended to predict the dynamical behaviour of supercooled liquids in general.

Free energy surface of supercooled water

We present a detailed analysis of the free energy surface of a well characterized rigid model for water in supercooled states. We propose a functional form for the liquid free energy, supported by recent theoretical predictions [Y. Rosenfeld and P. Tarazona, Mol. Phys. 95, 141 (1998)], and use it to locate the position of a liquid-liquid critical point at T(C')=130+/-5 K, P(C')=290+/-30 MPa, and rho(C')=1.10+/-0.03 g/cm(3). The observation of the critical point strengthens the possibility that the extended simple point charge model of water may undergo a liquid-liquid phase transition. Finally, we discuss the possibility that the approach to the liquid-liquid critical point could be pre-empted by the glass transition.