NONEQUILIBRIUM MOLECULAR-DYNAMICS CALCULATION OF THE THERMAL-DIFFUSION FACTOR

A nonequilibrium molecular dynamics method that allows direct calculation of the thermal diffusion factor, alpha, is presented. Using boundary conditions to establish a temperature gradient between two opposing faces of a cubic volume, a steady state is achieved in which the temperature and composition fields are spatially nonuniform. The thermal diffusion factor is obtained from observations of the temperature, composition and their gradients. We report calculations of alpha for a binary (isotopic) mixture of particles that interact with a (12-6) Lennard-Jones potential that is modified so that the interparticle force tends smoothly to zero at a finite range; supercritical systems of 108, 256, 500, and 1372 particles at a moderate fluid density are examined in which the particle mass ratios are 2, 5, and 10. We find that the dependence of alpha on the mass ratio (at fixed density, composition, and temperature) is similar to that observed in the Enskog theory for hard spheres.