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Molecular dynamics comparative study of Lennard-Jones -6 and exponential -6 potentials: application to real simple fluids (viscosity and pressure).

In this work, using molecular dynamics simulation, the viscosity (dynamic property) and the pressure (static property) of spherical fluid particles interacting through Lennard-Jones -6 and exponential -6 potentials are computed. Simulations are performed for going from 10 to 20 for the Lennard-Jones potential and from 12 to 22 for the exponential one. Six different thermodynamic states are tested that cover a large range of conditions, from sub- to supercritical temperature and from low to high density. To compare in a consistent manner the results for the various potentials tested, the simulations are carried out for the same set of reduced thermodynamic conditions (using the critical point). It is found that a perfect corresponding-states formulation is not possible between these potentials. Then, these potentials are applied on real simple fluids (argon, oxygen, nitrogen, methane, ethane, and one mixture, air) and the calculated viscosity and pressure values are compared with reference values. It appears that, using the appropriate , both potential families lead to a good accuracy in pressure and viscosity using the same set of molecular parameters for both properties, the average absolute deviations being always lower than 5% for the studied states. In addition, it is shown that the exponential potential results do not outperform the Lennard-Jones ones. Furthermore, for all compounds except for methane, the best results are obtained for the Lennard-Jones 12-6 and the exponential 14-6 potentials. This result partly explains why, despite no theoretical background, the Lennard-Jones 12-6 potential is so widely used. Finally, it is shown that a van der Waals one-fluid model performs extremely well for the studied mixture (air).