MONTE-CARLO CALCULATIONS ON THE BETA-DELTA PHASE-TRANSITION IN NITROGEN WITH A GENERALIZED FREE-ENERGY METHOD

Monte Carlo simulations that utilize an (N, P, T) ensemble with period ic deformable boundary conditions cannot describe phase transitions pr operly when a large potential barrier is involved. An alternative meth od is to calculate the Gibbs free energy difference between phases; th e transition occurs when the difference is equal to zero. The Gibbs fr ee-energy difference can be calculated using a generalized free-energy method. This method is used to determine the beta-delta phase transit ion of solid nitrogen at room temperature. The Gibbs free-energy diffe rence between the beta and the delta phase was obtained at 4.0 GPa. Th e difference at other pressures could be determined with the equation of state. The transition pressure was found at about 6.2 GPa, 1.3 GPa above the experimental pressure.