Mean properties and Free Energy of a few hard spheres confined in a spherical cavity

TL;DR

This study combines analytical and simulation methods to analyze the properties and free energy of a few hard spheres confined in a spherical cavity, covering low to high-density regimes.

Contribution

It provides new analytical and simulation insights into the thermodynamics and structural properties of small hard sphere systems in spherical confinement.

Findings

Density profiles vary with density and confinement.

Pressure and contact density match analytical results.

Free energy for four particles is estimated using a combined approach.

Abstract

We use analytical calculations and event-driven molecular dynamics simulations to study a small number of hard sphere particles in a spherical cavity. The cavity is taken also as the thermal bath so that the system thermalizes by collisions with the wall. In that way, these systems of two, three and four particles, are considered in the canonical ensemble. We characterize various mean and thermal properties for a wide range of number densities. We study the density profiles, the components of the local pressure tensor, the interface tension, and the adsorption at the wall. This spans from the ideal gas limit at low densities to the high-packing limit in which there are significant regions of the cavity for which the particles have no access, due the conjunction of excluded volume and confinement. The contact density and the pressure on the wall are obtained by simulations and compared to exact analytical results. We also obtain the excess free energy for N=4, by using a simulated-assisted approach in which we combine simulation results with the knowledge of the exact partition function for two and three particles in a spherical cavity.