Revisiting the phase diagram of hard ellipsoids

TL;DR

This study revisits the phase diagram of hard ellipsoids using replica exchange Monte Carlo simulations, revealing new insights into dense phases and transitions without imposing structural constraints.

Contribution

It introduces a simulation approach that better samples dense systems, uncovering phases and transitions not previously characterized in the classic phase diagram.

Findings

Identified plastic solids and fcc-like crystals at high densities.

Discovered SM2 structures for certain prolate and oblate ellipsoids.

Observed absence of transitions in small systems and reluctance of oblate cases to order.

Abstract

In this work the well-known Frenkel-Mulder phase diagram of hard ellipsoids of revolution [Mol. Phys. 55, 1171 (1985)] is revisited by means of replica exchange Monte Carlo simulations. The method provides good sampling of dense systems and so, solid phases can be accessed without the need of imposing a given structure. At high densities, we found plastic solids and fcc-like crystals for semi-spherical ellipsoids (prolates and oblates), and SM2 structures [Phys. Rev. E 75, 020402 (2007)] for x:1-prolates and 1:x-oblates with x>=3. The revised fluid-crystal and isotropic-nematic transitions reasonably agree with those presented in the Frenkel-Mulder diagram. An interesting result is that, for small system sizes (100 particles), we obtained 2:1 and 1.5:1-prolate equations of state without transitions, while some order is developed at large densities. Furthermore, the symmetric oblate cases are also reluctant to form ordered phases.