(735b) Computational Studies of Homogeneous Nucleation of Ice in Molecular Models of Water

Water is undoubtedly one of the most important molecules on earth, and is present in  many environments on our planet. In many of these environments, freezing of water- or lack thereof- is consequential. However, the freezing of water can be successfully suppressed for temperatures as low as 40 K below its melting point, which is due to large free energy barriers that separate the metastable liquid basin from the stable crystalline basin. It is therefore impossible to fully understand and predict the behavior of systems containing supercooled water without having any knowledge about the rate and the mechanisms of the freezing in those systems. Molecular simulations are invaluable tools in this endeavor, since the existing experimental techniques lack the spatiotemporal resolutions necessary for identifying the molecular-level events that lead to crystallization. However, the problem of calculating the rate of homogeneous nucleation of ice is one of the most challenging open problems in computational statistical physics, besting the efforts of generations of computational scientists [1]. Indeed, direct calculations of nucleation rates have only been done for mW, a monoatomic coarse-grained model of water, [2,3] and indirect estimates have been obtained for more sophisticated models of water using certain a priori assumptions [4] (e.g. the validity of the classical nucleation theory). To this date, no direct calculation has been reported for fully atomistic models of water, like the TIP4P family of models.  In this work, we employ a coarse-grained version of the forward-flux sampling (FFS) algorithm [5] to perform the first direct calculation of the homogeneous nucleation rate in a fully atomistic model of water. We also elaborate on the mechanism of freezing in the corresponding system. 

[1] Brukhno, Anwar, et al., J. Phys.: Condens. Matter 20, 494243 (2008).

[2] Li, Donadio, et al., Phys. Chem. Chem. Phys. 13, 19807 (2011).

[3] Li, Donadio, et al., Nat. Comm. 4, 1887 (2013).

[4] Sanz, Vega, et al. J Am. Chem. Soc. 135, 15008 (2013).

[5] Allen, Frenkel, et al., J. Chem. Phys. 124, 194111 (2006).