Flexible polymer structure requires simulation in the osmotic ensemble (using Monte Carlo or hybrid MD-MC schemes). The self-diffusion coefficient is calculated based on the Einstein relation from equilibrium MD, using the Mean Squared Displacement (MSD) of molecules over time. According to solution-diffusion theory, permeability is obtained by multiplying solubility and diffusivity (based on self-diffusion), and perm-selectivity is calculated as the ratio of the permeabilities of two gas components. This approach has been widely adopted in several studies, including Jiang and co-workers, Bhatia and co-workers, and many other researchers in this field. Furthermore, Keskin and co-workers have applied this strategy to the screening of Metal-Organic Frameworks (MOFs) as membrane materials.