The barrier to ice nucleation in monatomic water

TL;DR

This paper investigates the nucleation barrier of ice formation in supercooled monatomic water, highlighting how different particle classification methods influence the perceived nucleation process and the importance of suitable reaction coordinates.

Contribution

It compares various protocols for identifying solidlike particles in monatomic water, revealing their impact on nucleation barrier shape, height, and cluster structure, and discusses the limitations of simple collective variables.

Findings

Nucleation barrier shape and size are affected by particle classification methods.

The dominant ice phase in nucleation depends on the classification protocol.

Reaction coordinate quality cannot be judged solely by barrier height.

Abstract

Crystallization from a supercooled liquid initially proceeds via the formation of a small solid embryo (nucleus), which requires surmounting an activation barrier. This phenomenon is most easily studied by numerical simulation, using specialized biased-sampling techniques to overcome the limitations imposed by the rarity of nucleation events. Here, I focus on the barrier to homogeneous ice nucleation in supercooled water, as represented by the monatomic-water model, which in the bulk exhibits a complex interplay between different ice structures. I consider various protocols to identify solidlike particles on a computer, which perform well enough for the Lennard-Jones model, and compare their respective impact on the shape and height of the nucleation barrier. It turns out that the effect is stronger on the nucleus size than on the barrier height. As a by-product of the analysis, I determine the structure of the nucleation cluster, finding that the relative amount of ice phases in the cluster heavily depends on the method used for classifying solidlike particles. Moreover, the phase which is most favored during the earlier stages of crystallization may happen, depending on the nucleation coordinate adopted, to be different from the stable polymorph. Therefore, the quality of a reaction coordinate cannot be assessed simply on the basis of the barrier height obtained. I explain how this outcome is possible and why it just points out the shortcoming of collective variables appropriate to simple fluids in providing a robust method of particle classification for monatomic water.