However, this strategy has notable limitations, which overlap with challenges seen in theoretical and experimental approaches. First, it depends on the validity of the solutiondiffusion theory, and like experiments, it assumes ideal perm-selectivity based on the independent behavior of the two components. Other challenges include the inability of GCMC simulations to distinguish between accessible porosity and isolated pores and the fact that solubility and diffusivity are typically calculated in a single, specific loading regime (e.g. the Henry's law regime of the adsorption isotherm). In an actual membrane, concentration of diffusing species and transport mechanisms vary across the membrane, and these properties generally need to be averaged across the membrane to be meaningful in the solution-diffusion theory context. Furthermore, while self-diffusion coefficients can be reliably obtained in crystalline materials such as Metal-Organic Frameworks (MOFs) and zeolites, they are challenging to measure in disordered porous media such as polymers. Many reported MD simulations suffer from trajectories that are too short and noisy for consistent interpretation. For example, to overcome this challenge, in the studies by Fayaz-Torshizi et al., 60 independent runs were required to obtain a reliable MSD trend. Due to these difficulties, some studies have abandoned traditional diffusion calculations in polymers altogether, opting instead for semi-empirical methods to estimate diffusivity. The second approach relies on non-equilibrium molecular dynamics (NEMD), where the flux of a single component or multicomponent mixture is directly simulated across a model membrane. In the original method, Dual Control Volume Grand Canonical Molecular Dynamics (DCVGCMD), the system contains two distant regions, in which their chemical potentials are maintained by exchanging particles with their respective bulk reservoir. Outside these regions, constant-temperature molecular dynamics governs particle behavior.