The tendency of molecules to move in response to a concentration gradient is described by the transport diffusion coefficient. In molecular simulations, self-diffusion coefficient is often considered as an approximation of the transport diffusivity. This approach is reasonable in low concentration regimes, as in the zero loading limit the self-diffusion coefficient is equal to transport diffusion coefficient. The self-diffusion coefficient is commonly obtained using the Einstein relation based on the Mean Square Displacement (MSD) of the gas molecules as a function of time. In the SI we provide analysis using this approach and discuss the challenges associated with the reliable calculation of MSD in dense polymer systems.