C60sphase diagram: A full free-energy analysis

The phase diagram of nonrigid spheres with pair potential chosen to mimic the ${\mathrm{C}}_{60}$-${\mathrm{C}}_{60}$ interaction was studied by isothermal-isobaric molecular-dynamics simulation. The crystal phase was explored from zero pressure up to 12 000 bars, while the liquid, vapor, and fluid phases were studied up to 600 bars. By combining the resulting enthalpic data with a full harmonic analysis for the crystal and an integration of the fluid phase to infinite temperature to reach the ideal-gas limit, the free energies of the four competing phases were obtained. In accord with experimental observation and previous Gibbs-ensemble and Clausius-Clapeyron simulation methods, the liquid phase is found to be unstable everywhere in the phase diagram. Comprehensive enthalpic, density, and free-energy fits to the data are given. As more reliable ${\mathrm{C}}_{60}$ interactions become available, the present model has the potential to become a highly accurate reference system. Contrary to common orthodoxy, it is possible via the direct molecular-dynamics methods described herein to distinguish the thermodynamics of dense states separated by only a few atmospheres.