Sublimation of the Modified Lennard-Jones System

By adopting the isothermal-isochoric ensemble instead of the isothermal-isobaric one, the density of the modified Lennard-Jones system could be specified unambiguously even under extremely low-pressure conditions. The Gibbs free energy of the gaseous state could thereby be calculated with sufficient accuracy. For practical applications of the free energy, the lowest correction in the virial expansion was found to be sufficient under pressure conditions below 0.001 epsilon/sigma(3), where epsilon and sigma are the energy and length parameters, respectively, of the modified Lennard-Jones potential. Comparison of this free energy with those for the condensed phases obtained hitherto gave well-defined sublimation points. These points then defined the equilibrium solid-gas phase boundary. The present study thus confirmed the location of not only a sublimation point found previously under marginally stable conditions of an isobaric ensemble, but also the triple point; its temperature and pressure were speculated to be 0.61 epsilon/k(B) and 0.0018 +/- 0.0005 epsilon/sigma(3), respectively, where k(B) denotes Boltzmann's constant.