SOLID-FLUID EQUILIBRIA FOR HARD DUMBBELLS VIA MONTE-CARLO SIMULATION

We present the results of a study of solid-fluid phase equilibria in systems of hard dumbbells for three values of the dumbbell bond length. Monte Carlo simulations were used to calculate the equation of state and Helmholtz free energy. Four orientationally ordered solid phases have been considered-the alpha-N2 structure and three different base centered monoclinic structures formed by the stacking of hexagonally packed layers that allow the dumbbells to achieve their maximum packing density. In addition, a face-centered-cubic (fcc) plastic crystal was studied for the system with the lowest bond length. The three base centered monoclinic structures have thermodynamic properties which are indistinguishable at the level of accuracy in our calculations. For longer bond lengths, the stable solid structure tends to be orientationally ordered base centered monoclinic. However, we also consider the stability of an aperiodic crystal for the case of dumbbells formed from tangent spheres. At lower bond lengths, the system freezes into a fcc plastic crystal which becomes unstable with respect to a base centered monoclinic structure at higher pressure. The transition between these solid phases is apparently first order. The behavior for the lowest bond length considered resembles that of nitrogen at high temperatures. Our results suggest that the alpha-N2 is not a stable crystal structure for hard dumbbell solids at any bond length, but does appear as a metastable phase in some cases.