Molecular understanding of diffusion in confinement

We introduce a computational method to directly relate diffusivities to the microscopic behavior of the adsorbed molecules. We apply this method to gases in an MFI-type molecular sieve, the reference system in this field. Transitions in the number and nature of adsorption sites result in temporary local increases in the diffusion. This occurs at different loadings in each of the x, y, and z directions, giving rise to the complex loading behavior found experimentally. Our method can be applied to any adsorbent-adsorbate system, and provides a fundamental understanding of diffusion in confinement on a molecular level.