Free energy determination of phase coexistence in model C60: A comprehensive Monte Carlo study

TL;DR

This study uses extensive Monte Carlo simulations to accurately determine the free energy and phase diagram of the Girifalco C60 model, revealing the stability of a narrow liquid pocket and providing valuable reference data for fullerene materials.

Contribution

It presents a comprehensive Monte Carlo approach to calculate free energies and phase coexistence in the C60 model, including phase diagram details and validation of the liquid pocket stability.

Findings

Confirmed the existence of a narrow liquid pocket in the phase diagram.

Critical temperature closely follows an extended corresponding-states rule.

Provided reference free energy data for fullerene-related materials.

Abstract

The free energy of the solid and fluid phases of the Girifalco C60 model are determined through extensive Monte Carlo simulations. In this model the molecules interact through a spherical pair potential, characterized by a narrow and attractive well, adjacent to a harshly repulsive core. We have used the Widom test particle method and a mapping from an Einstein crystal, in order to estimate the absolute free energy in the fluid and solid phases, respectively; we have then determined the free energy along several isotherms, and the whole phase diagram, by means of standard thermodynamic integrations. We highlight how the interplay between the liquid-vapor and the liquid-solid coexistence conditions determines the existence of a narrow liquid pocket in the phase diagram, whose stability is assessed and confirmed in agreement with previous studies. In particular, the critical temperature follows closely an extended corresponding-states rule recently outlined by Noro and Frenkel [J. Chem. Phys. 113:2941 (2000)]. We discuss the emerging "energetic" properties of the system, which drive the phase behavior in systems interacting through short-range forces [A. A. Louis, Phil. Trans. R. Soc. A 359:939 (2001)], in order to explain the discrepancy between the predictions of several structural indicators and the results of full free energy calculations, to locate the fluid phase boundaries. More generally, we aim to provide extended reference data for calculations of the free energy of the C60 fullerite in the low temperature regime, as for the determination of the phase diagram of higher order fullerenes and other fullerene-related materials, whose description is based on the same model adopted in this work.