Quantifying Large Effects of Framework Flexibility on Diffusion in MOFs: CH4 and CO2 in ZIF‐8

: Breathe in, breathe out: efficient methods are introduced for assessing the role of framework flexibility on molecular diffusion in metal-organic frameworks (MOFs) that does not require defining a classical forcefield for the MOF. These methods combine ab initio MD of the MOF with classical MD simulation of the diffusing molecules. The effects of flexibility are shown to be large for CH(4), but not for CO(2), in ZIF-8.