Statistical mechanics of crystal nuclei of hard spheres

Abstract: In the study of crystal nucleation via computer simulations, hard spheres are arguably the most extensively explored model system. Nonetheless, even in this simple model system, the complex thermodynamics of crystal nuclei can sometimes give rise to counterintuitive results, such as the recent observation that the pressure inside a critical nucleus is lower than that of the surrounding fluid, seemingly clashing with the strictly positive Young--Laplace pressure we would expect in liquid droplets. Here, we re-derive many of the founding equations associated with crystal nucleation, and use the hard-sphere model to demonstrate how they give rise to this negative pressure difference. We exploit the fact that, in the canonical ensemble, a nucleus can be in a (meta)stable equilibrium with the fluid, and measure the surface stress for both flat and curved interfaces. Additionally, we explain the effect of defects on the chemical potential inside the crystal nucleus. Lastly, we present a simple, fitted thermodynamic model to capture the properties of the nucleus, including the work required to form critical nuclei.