Non-destructive molecular-dynamics simulation of the chemical potential of a fluid

We have evaluated the chemical potential by molecular-dynamics simulation for a Lennard-Jones (L-J) fluid and for a Lennard-Jones shifted-force (L-J, sf) fluid over a wide range of temperature and density by Widom's 6 particle-insertion-energy method. We have also investigated, in some detail, more recent methods 10, 11 using the energy of a real particle, and combinations of both methods using the energy-distribution functions. We find that these methods are accurate, convenient and economical but are no better than the straightforward Widom method. We confirm that the molecular-dynamics method is as good, if not better, than the corresponding Monte-Carlo techniques and the grand-canonical Monte-Carlo method. We present results for the properties of the L-J, sf3 fluid, in particular the liquid-vapour co-existence curve, including the critical point, and compare them with those for the L-J fluid and for liquid argon. The method is of general application and, in particular, may be directly used for molecular liquids and mixtures. It requires only a simple addition to standard molecular-dynamics programmes.