C-60 PHASE-DIAGRAM - A FULL FREE-ENERGY ANALYSIS

The phase diagram of nonrigid spheres with pair potential chosen to mi mic the C-60-C-60 interaction was studied by isothermal-isobaric molec ular-dynamics simulation. The crystal phase was explored from zero pre ssure up to 12 000 bars, while the liquid, vapor, and fluid phases wer e studied up to 600 bars. By combining the resulting enthalpic data wi th 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 wi th experimental observation and previous Gibbs-ensemble and Clausius-C lapeyron 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 C-60 interac tions become available, the present model has the potential to become a highly accurate reference system. Contrary to common orthodoxy, it i s possible via the direct molecular-dynamics methods described herein to distinguish the thermodynamics of dense states separated by only a few atmospheres.