Liquid-vapor coexistence in fluids of dipolar hard dumbbells and spherocylinders

Fluids of dipolar hard spheres are thought to have rather unusual phase behavior in that liquid-vapor equilibrium has not been observed in computer simulations. Recently, McGrother and Jackson [Phys. Rev. Lett. 76, 4183 (1996)] have reported Gibbs ensemble Monte Carlo (GEMC) results for dipolar hard spherocylinders. At the reduced temperature T*=0.12 they report liquid-vapor coexistence for aspect ratios L/D greater than or similar to 0.19, bur no coexistence was found for smaller values. In the present paper we investigate the phase behavior of dipolar hard dumbbells and dipolar hard spherocylinders using both GEMC and grand canonical Monte Carlo methods. For both models at the same reduced temperature we find liquid-vapor coexistence for aspect ratios as low as 0.1. For aspect ratios less than or similar to 0.1, the strong dipolar interactions create severe sampling problems and reliable results cannot be obtained. This is particularly true for the GEMC method and possibly accounts for the disagreement we find with the earlier simulations. [S1063-651X(99)08502-5].