(375g) Polymorph Selection in the Homogeneous Nucleation of Ice in Deeply Supercooled Water

Ice nucleation is a ubiquitous phenomenon in nature, with important consequences in a variety of biological, geological and atmospheric systems. However, the molecular mechanism of ice nucleation is difficult to discern in experiments. The ice that nucleates homogeneously in the atmosphere, and in vapor chamber experiments is the stacking-disordered ice that predominantly comprised of the cubic polymorph, and not the thermodynamically stable hexagonal polymorph [1]. This preference for the less stable polymorph in nucleation has been broadly rationalized using the Ostwald step rule [2]. However, the molecular origin of this preference is unknown. In this work, we use an advanced sampling technique, known as forward-flux sampling to study the kinetics and mechanism of ice nucleation in molecular (TIP4P/Ice) and coarse-grained (mW) models of water. By using a novel topological approach [3], we explain this preference in terms of different growth propensities of different topological features that are consistent with cubic and hexagonal polymorphs. We also investigate the nucleation pathways for the exclusive nucleation of each of these polymorphs. 

[1] Malkin et al., Proc. Natl. Acad. Sci. 109: 1041 (2012).

[2] Malkin et al., Phys. Chem. Chem. Phys. 17: 60-76 (2015).

[3] Haji-Akbari, Debenedetti, Manuscript Under Review (arxiv:1505.01126).