Direct Gibbs ensemble Monte Carlo simulations for solid-vapor phase equilibria: Applications to Lennard-Jonesium and carbon dioxide

The Gibbs ensemble Monte Carlo method of Panagiotopoulos is extended to cal culations of solid-vapor coexistence curves. As in the original Gibbs ensem ble method, the new technique makes use of two simulation boxes that are in thermodynamic contact. However, the box that contains the solid phase is e longated along one axis and contains only a slab of solid material surround ed on both sides by vapor. Aggregation-volume-bias Monte Carlo moves are us ed to sample transfers from the solid to the vapor and vice versa in this b ox, whereas the usual particle swap moves are applied to transfers between the solid-vapor box and the other box that contains a bulk vapor phase. Vol ume moves for the solid-vapor box use separate displacements of individual cell lengths or of individual R-matrix elements. As one approaches the trip le-point temperature from below, increased disorder at the solid-vapor inte rface is observed, and once the triple-point temperature is exceeded, the e ntire solid slab converts to a liquid, The use of configurational-bias Mont e Carlo particle swap moves enables us to extend conventional Gibbs ensembl e simulations of vapor-liquid equilibria beyond the triple point into the s upercooled regime. Clausius-Clapeyron fits to the sublimation, and vapor pr essure curves allow for the precise determination of the triple-point locat ion. The simulation results for Lennard-Jonesium are in excellent agreement with Gibbs-Duhem integration simulations, and the results for carbon dioxi de using the TraPPE force field reproduce well the experimental data (e.g., the predicted triple-point parameters are T = 212 +/- 2 K and 430 +/- 50 k Pa),