We set up a discrete space-time dynamical model of molecules with thermalised kinetic energy and repulsive cores, in an external potential. The state is specified by a probability on the sample space. One time-step is given by a bistochastic map, followed by a local thermalising map. The model obeys the first and second laws of thermodynamics. The continuum limit, obtained using a MAPLE program, gives rise to coupled nonlinear reaction-diffusion equations for the density and temperature fields. The system obeys Onsager symmetry and exhibits the Soret and Dufour effects.

Source: LATEX. King's College London. The MAPLE program given here will not appear in the published version (in Proc Roy Soc)