Relationship between the line of density anomaly and the lines of melting, crystallization, cavitation, and liquid spinodal in coarse- grained water models

JB Lu and C Chakravarty and V Molinero, JOURNAL OF CHEMICAL PHYSICS, 144, 234507 (2016).

DOI: 10.1063/1.4953854

Liquid water has several anomalous properties, including a non- monotonous dependence of density with temperature and an increase of thermodynamic response functions upon supercooling. Four thermodynamic scenarios have been proposed to explain the anomalies of water, but it is not yet possible to decide between them from experiments because of the crystallization and cavitation of metastable liquid water. Molecular simulations provide a versatile tool to study the anomalies and phase behavior of water, assess their agreement with the phenomenology of water under conditions accessible to experiments, and provide insight into the behavior of water in regions that are challenging to probe in the laboratory. Here we investigate the behavior of the computationally efficient monatomic water models mW and mTIP4P/2005(REM), with the aim of unraveling the relationships between the lines of density extrema in the p-T plane, and the lines of melting, liquid-vapor spinodal and non- equilibrium crystallization and cavitation. We focus particularly on the conditions for which the line of density maxima (LDM) in the liquid emerges and disappears as the pressure is increased. We find that these models present a retracing LDM, same as previously found for atomistic water models and models of other tetrahedral liquids. The low-pressure end of the LDM occurs near the pressure of maximum of the melting line, a feature that seems to be general to models that produce tetrahedrally coordinated crystals. We find that the mW water model qualitatively reproduces several key properties of real water: (i) the LDM is terminated by cavitation at low pressures and by crystallization of ice I-h at high pressures, (ii) the LDM meets the crystallization line close to the crossover in crystallization from ice I-h to a non-tetrahedral four-coordinated crystal, and (iii) the density of the liquid at the crossover in crystallization from ice I-h to a four-coordinated non- tetrahedral crystal coincides with the locus of maximum in diffusivity as a function of pressure. The similarities in equilibrium and non- equilibrium phase behavior between the mW model and real water provide support to the quest to find a compressibility extremum, and determine whether it presents a maximum, in the doubly metastable region. Published by AIP Publishing.

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