Molecular modeling and simulation offer a complimentary approach to gain a deeper understanding of the microscopic mechanisms of thermal diffusion and also allow for the prediction of Soret and other transport coefficients. The Lennard-Jones interaction potential is particularly attractive because it is a computationally efficient and yet realistic molecular model, thus helping to rapidly generate a fundamental understanding of molecular processes. Furthermore, Lennard-Jones mixtures can exhibit a rich vapor-liquid equilibrium behavior, which can facilitate constructing new theoretical models and testing applica-tion limits of existing ones. Knowing the phase diagram is in itself central to studying thermodiffusion because phase changes are typically to be avoided, that is, gas bubbles and solid precipitates are unwanted when investigating thermal diffusion in the liquid state.