KMC Simulation of the Thermal Diffusion of Binary Lennard-Jones Systems

Prasanth Sankar and Shawn A. Putnam

UIUC Matse385

December 2001

Abstract:

The objective of this project was to gain a better understanding of the coupling mechanisms between the temperature and concentration fields in the phenomena known as thermal diffusion or the (Ludwig-Soret) effect. We simulated binary Lennard-Jones systems using a driven Kinetic Monte Carlo method, where the lattice was be subjected to a temperature gradient. One reason why this problem is interesting is because recent experimental advances are allowing measurements of the thermal diffusion constant in a wide range of systems, starting with simple binary fluids to polymer solutions. A second reason is that all of the currently available simulation methods are based on molecular dynamics and are not only extremely time consuming but questionable. In addition, the applicability of Monte Carlo methods to such problems has not yet been tested. We have shown via a Transition State Theory (TST) approach that in achievable computer time a Binary Lennard-Jones system can be driven to a nonequilibrium steady state. In general that the Kinetic Monte Carlo methods are not only applicable but efficient in simulating such problems. In addition our results, which are comparable to the results of past MD simulations, show that it is sufficient to capture the actual coupling mechanism between the thermal and concentration fields with the assumption that the local rate constants (in conjunction with Transition State Theory) are a function of local temperature.