Phases of supercooled liquid water

Simulations essentially extend possibility to study supercooled liquid water, as crystallization may be suppressed. However, there is no water model, which adequately reproduces phase diagram of water and its properties even in the thermodynamic region, where experimental data are available. In such situation, only comparative analysis of the results, obtained for various water models, can give information, relevant for the behaviour of real water in supercooled region. Additional complication appears due to the necessity to use sophisticated simulation methods, appropriate for the studies of the phase transitions, such as Monte Carlo simulations in the grand canonical or in the Gibbs ensemble (see Refs.7,16 for more details). Note, that simulations in the simple constant-volume or constant-pressure ensembles, widely used in the studies of supercooled water (see, for example Refs. 17,18), are not appropriate for the location of the phase transitions. 

In this paper we compare isotherms of supercooled water, obtained in simulations of various water models, with available experimental data. Special attention is focused on the first (lowest-density) liquid-liquid transition, which influences properties of liquid water at zero pressure. This liquid-liquid transition of water is considered in terms of increasing concentration and percolation transition of the four-coordinated water molecules upon cooling. 

The pressure-density isotherms of various water models in supercooled region, obtained in simulations, can be directly compared with the available experimental isotherms, showing transformations between amorphous ices upon compression. Experimental measured isotherms are unavoidably affected by the transformation kinetics and by the strong hysteresis. These two effects may be reduced by using slow compression rates and higher temperatures, respectively. Therefore, the equilibrium isotherms obtained in simulations we compare with the experimental pressurization curves, which were obtained under the slowest compression rate and at highest temperature. 

There are four phases of supercooled ST2 water, which densities at corresponding liquid-liquid coexistence in the temperature interval from about 200 to 235 K are  

10 and p 1.20 g/cm. Obviously, water phase I is an analogue of EDA. It is important, that phase 1 of ST2 water and EDA exist at zero pressure. Most of water molecules in this phase are four-coordinated and show perfect tetrahedral ordering of the nearest four neighbours. First and second coordination shells are well defined in phase 

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I. Brovchenko, A. Oleinikova, Phases of supercooled liquid water, in W. Kuhs (ed.) Proceedings of the 11th International Conference on the Physics and Chemistry of Ice, Cambridge, England Cambridge Royal Society of Chemistry, 2007, pp. 117-124. 

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