Comparison of Diffusion Models: Transition State Theory vs. Langevin Equation Marc Dvorak Department of Physics, Colorado School of Mines Date: Oct. 24, 2011 I will outline the differences in theory and results for diffusion calculations based on transition state theory (TST) and a Langevin equation approach. In the context of impurity diffusion through a crystalline material, transition state theory assumes an Arrhenius form for the jump rate of the impurity between two interstitial sites. The two parameters for the Arrhenius form - activation energy and prefactor - can be estimated with density functional theory (DFT). The more rigorous Langevin equation approach explicitly includes a friction term between the impurity and the substrate, which can also be estimated with DFT. It can be shown that TST is a particular limit of the Langevin approach. Our goal is to compare diffusion rates predicted by the two theories, in particular by studying the diffusion of hydrogen through diamond. Limited results for this diamond system will be shown.