Cause and effect reversed in non-equilibrium molecular dynamics: an easy route to transport coefficients

Abstract

A novel non-equilibrium method for calculating transport coefficients is presented. It reverses the experimental cause-and-effect picture, e.g. for the calculation of viscosities: the effect, the momentum flux or stress, is imposed, whereas the cause, the velocity gradient or shear rates, is obtained from the simulation. It differs from other Norton-ensemble methods by the way, in which the steady-state fluxes are maintained. This method involves a simple exchange of particle momenta, which is easy to implement and to analyse. Moreover, it can be made to conserve the total energy as well as the total linear momentum, so no thermostatting is needed. The resulting raw data are robust and rapidly converging. The method is tested on the calculation of the shear viscosity, the thermal conductivity and the Soret coefficient (thermal diffusion) for the Lennard–Jones (LJ) fluid near its triple point. Possible applications to other transport coefficients and more complicated systems are discussed.