Using Molecular Simulation to Compute Transport Coefficients of Molecular Gases

The existing kinetic theory of gases is based on an analytical approach that becomes intractable for all but the simplest molecules. Here we propose a simple numerical scheme to compute the transport properties of molecular gases in the limit of infinite dilution. The approach that we propose is approximate, but our results for the diffusivity D, the viscosity eta, and the thermal conductivity lambda of hard spheres, Lennard-Jones particles, and rough hard spheres agree well with the standard (lowest order) Chapman-Enskog results. We also present results for a Lennard-Jones-dimer model for nitrogen, for which no analytical results are available. In the case of polyatomic molecules (we consider n-octane), our method remains simple and gives good predictions for the diffusivity and the viscosity. Computing the thermal conductivity of polyatomic molecules requires an approximate treatment of their quantized internal modes. We show that a wellknown approximation that relates lambda to D and eta yields good results. We note that our approach should yield a lower limit to the exact value of D, eta, and lambda. Interestingly, the most sophisticated (higher-order) Chapman-Enskog results for rough hard spheres seem to violate this bound.