The dependence of the phase diagram on the range of the attractive intermolecular forces

A density functional theory of freezing combined with a thermodynamically consistent integral equation method is used to investigate the phase behaviour of systems interacting via the m-n potential with m = 2n, phi(r) = 4 epsilon[(sigma/r)(2n) - (sigma/r)(n)] (n = 6, 8, 10 and 12) and rigid C-60 molecules interacting via the Girifalco potential. It is found that the liquid-vapour coexistence region is gradually suppressed as the attractive part of the potential becomes short range with increasing n and the coexistence ceases to occur at n approximate to 11. The m-n potential with n = 11-12 is similar to the Girifalco potential and the two yield similar phase diagrams. It is also found that the phase diagram of C-60 calculated for a truncated potential is qualitatively in agreement with the corresponding Monte Carlo (MC) simulations of Hagen et al, which have predicted nonexistence of the liquid phase in contrast to the molecular dynamics (MD) simulations of Cheng et al. These results suggest the importance of treating the long-range tail of the potential correctly and provide a partial explanation for the discrepancy between the MC and MD simulations.