Direct Calculation of Ice Homogeneous Nucleation Rate for a Molecular Model of Water

TL;DR

This study presents the first direct calculation of ice nucleation rates in a molecular water model using path sampling, revealing a competitive mechanism favoring cubic ice formation over hexagonal ice.

Contribution

It introduces a novel topological approach and applies it to perform the first direct rate calculation for molecular water, elucidating the nucleation mechanism.

Findings

Cubic ice tends to dominate early nucleation stages.

Transition states are rich in cubic ice motifs.

The mechanism explains the experimental preference for cubic ice.

Abstract

Ice formation is ubiquitous in nature, with important consequences in a variety of environments, including biological cells, soil, aircraft, transportation infrastructure and atmospheric clouds. However, its intrinsic kinetics and microscopic mechanism are difficult to discern with current experiments. Molecular simulations of ice nucleation are also challenging, and direct rate calculations have only been performed for coarse-grained models of wate. For molecular models, only indirect estimates have been obtained, e.g. by assuming the validity of classical nucleation theory. We use a path sampling approach to perform the first direct rate calculation of homogeneous nucleation of ice in a molecular model of water. We use TIP4P/Ice, the most accurate among existing molecular models for studying ice polymorphs. By using a novel topological approach to distinguish different polymorphs, we are able to identify a freezing mechanism that involves a competition between cubic and hexagonal ice in the early stages of nucleation. In this competition, the cubic polymorph takes over since the addition of new topological structural motifs consistent with cubic ice leads to the formation of more compact crystallites. This is not true for topological hexagonal motifs, which give rise to elongated crystallites that are not able to grow. This leads to transition states that are rich in cubic ice, and not the thermodynamically stable hexagonal polymorph. This mechanism provides a molecular explanation to the earlier experimental and computational observations of the preference for cubic ice in the literature.