Computer simulation water glass transition

The proximity of this liquid-liquid transition to the protein-glass transition temperature is suggestive. Clearly, at temperatures below 220 K or so, the dynamics of water and protein are highly coupled. A recent computer simulation study has shown that the stmctural relaxation of protein requires relaxation of the water HB network and translational displacement of interfacial water molecules. It is, therefore, clear that the dynamics of water at the interface can play an important role. This is an interesting problem that deserves further investigation. 

In this work, we review briefly the phenomenology associated to LLPTs based on results obtained from computer simulations of different systems, such as silica, water, and atomic model systems. When possible, results from computer simulations are compared to available experiments. This work is organized as follows. In the next section, we present the phase diagram of polymorphic liquids supported by many computer simulations and experiments. We review the thermodynamics of first-order phase transitions and show how it is observed in computer Simula tions of polymorphic liquids. The relationship between liquid polymorphism and anomalous properties in liquids is also discussed. The next section also includes a description of glass polymorphism, its relation to liquid polymorphism, and a close comparison between experiments and simulations. In Section III, we describe computer simulation models of systems that present liquid polymorphism, with emphasis on the molecular interactions and common properties of these models that are thought to originate LLPTs. A summary and discussion are presented in Section IV. 

Multicomponent systems that present polyamorphism have also been reported in computer simulation studies. For example, in Ref. [35], it is found that silica has a LLCP at very low temperature. Silica is also a tetrahedral liquid and it shares many of the thermodynamic properties observed in water. In Ref. [35], two silica models were considered. In both models, the interactions among O and Si atoms are isotropic, due to single point charges and short-range interacting sites located on each atom. Both models considered in Ref. [35] are characterized by a LLCP at very low temperature and coexistence between two liquids is observed in out of equilibrium simulations close to one of the spinodal lines (see Fig. 2b). The location of the LLCP was estimated to be below the glass transition in real silica and hence, unaccessible in experiments. We note that polyamorphism in the glass state is indeed observed in compression experiments on amorphous silica [14], and is qualitatively reproduced in computer simulations [89]. Other examples of multicomponent systems that show LLPT in simulations are presented in Refs [65,90]. In these cases, a substance that already shows polymorphism is mixed with a second component. 

---