The results of molecular dynamics calculations of thermal rates of diffusion of C and H atoms in Ar matrices were presented. The computed results were compared with those computed by a simple transition-state theory. The results of the simple transition-state theory, which involved trivial computations, were in good agreement with the results of the classical molecular dynamics calculations, which required large amounts of computer time. These results showed that the simple transition-state theory was adequate for predicting thermal rates of diffusion. The applicability of simple transition-state theory to diffusion in matrices provided an easy method for predicting rates since it greatly reduced the amount of information needed about the potential and effort in computation compared with methods such as Monte Carlo variational transition-state theory and molecular dynamics calculations. Calculations using simple transition-state theory were also carried out for O- and H-atom diffusion in xenon. The computed rates were smaller than the experimental values, indicating that the experimentally observed processes were likely occurring in matrices with imperfections, as was suggested previously by others.Molecular Dynamics and Simple Transition-State Theory Predictions of Rates of Atomic Diffusion in Rare-Gas Matrices. Guo, Y., Thompson, D.L.: The Journal of Chemical Physics, 1995, 103[20], 9024-9