PARTIAL SPECIFIC QUANTITIES COMPUTED BY NONEQUILIBRIUM MOLECULAR-DYNAMICS

Partial specific enthalpies were computed directly from a nonequilibrium molecular dynamics (NEMD) simulation with a nonzero composition gradient and zero temperature- and pressure gradients. The results agree with data from thermodynamic differentiation of equilibrium data to within statistical uncertainties. We report here results on a binary Lennard-Jones/spline isotope mixture with component mass ratio m(1)/m(2) equal to 10 and at overall reduced temperature and density equal to 2 and 0.1, respectively (in Lennard-Jones units). This is a thermodynamically ideal mixture, and the partial specific quantities follow trivially from data for the pure components. The numerical NEMD method will, however, work equally well for non-ideal mixtures, and the isotope mixture was used as a test of the NEMD method. An isothermal and isobaric system with a composition gradient was established with a particle swapping algorithm. Assuming that local thermodynamic equilibrium is obtained, this gives a composition profile through the system that enables us to obtain partial specific (or molar) quantities essentially as the ratio between the gradient of the quantity of interest and the composition gradient. Under the assumption that the Onsager reciprocal relations are valid, the chemical potential gradients of the two components were determined and found to be in good agreement with the ideal mixture result.