For the NEB computations, experimental crystal structures of both MFU-4 and MFU-4-Br were first optimized using the PBEsol functional with D3 dispersion correction , DZVP-MOLOPT-SR basis set, and the Goedecker-Teter-Hutter pseudopotentials . The optimized cubic unit cells with a resulting lattice constant of 21.61 Å for MFU-4 and of 21.66 Å for MFU-4-Br, and thereon based theoretical pore apertures of 2.48 Å and 2.19 Å, respectively, are in agreement with the experimental reference values given in Table . Based on the optimized unit cell structures, NEB computations were carried out with consistent computational setup, thus implying the PBEsol functional. The trajectory of the modeled gas diffusion path through the framework structure is exemplarily shown for Xe in MFU-4 in Figure : Starting from a structure where Xe is in the middle of the large void of 11.94 Å (left frame), the gas atom passes to the much smaller pore of 3.9 Å through a four-fold coordinated window of halogen atoms, in this case of chlorine atoms colored in green (middle frame). Passing in a reverse sense from the small to the large void, the Xe atom is finally back in the large void (right frame). The potential energy profiles of Figure correspond to the outlined NEB trajectory: The profile includes two peaks corresponding to the situations where the noble gas atom passes through the halogen window, going from either large to small pore (left peak) or from small to large pore (right peak). Activation energies from large pore to small pore are found to be largest and therefore represent the decisive activation barrier height. In summary, the results indicate an activation energy of 63.9 and 42.9 kJ•mol -1 for Xe and Kr gases in MFU-4, and of 89.0 and 88.5 kJ•mol -1 for Xe and Kr in MFU-4-Br, respectively (see also Table ). The PBEsol results thus imply an ordering and trend in activation energies that follows Kr&MFU-4 < Xe&MFU-4 < Kr&MFU-4-Br  Xe&MFU-4-Br, agreeing with the calculated guest to host ratios (Table ) and with experimental data. PBEsol barrier heights were furthermore assessed by increasing the accuracy to meta-GGA, hybrid and double-hybrid density functionals (DHDFs). To keep balance between methodological and basis-set incompleteness error, basis sets were enlarged to triplet-zeta quality. Due to the large unit cell size of the investigated MFU-4-MOFs, it is then no longer feasible to perform geometry optimizations or NEB computations even at hybrid functional level due to the drastically increasing computational costs. The following assessment is thus restricted to computing single-point energies based on the PBEsol optimized structures as obtained from the NEB computations. For the relatively cheap PBE0 hybrid and the rSCAN meta-GGA functional, single-point computations were performed along the entire minimum energy path. When switching to the more cost intensive double hybrid functionals ωB97X-2 and revDSD-PBEP86-D4 as well as when performing RPA computations, reassessment was restricted to few geometries corresponding to highest energy peaks and lowest energy valleys of the NEB paths. Resulting activation energies of Xe and Kr obtained from PBE0 single-point energies amount up to 81.6 and 50.7 kJ•mol -1 in MFU-4, and 100.9 and 98.1 kJ•mol -1 in MFU-4-Br, respectively. The energy profiles obtained from corresponding rSCAN calculations agree with the PBE0 functional results as displayed in Figure , with the former yielding slightly higher barrier energies than the latter. The rSCAN barriers were determined to be 75. two error ranges also reduce to a maximum of -16.1 and -13.1 kJ/mol -1 . However, if not comparing largest barriers, but relative energies to the PBEsol reference, both double hybrid and RPA computations indicate larger error ranges for Xe/MFU-4-Br, as visualized in Figure . Furthermore, it was found that the energy of the PBEsol-optimized geometry corresponding to the most favorable adsorption site is higher in energy than corresponding energies for the PBEsol-optimized geometries in the middle of the small pore for the ωB97X-2 results of Kr/MFU-4, Xe/MFU-4 and Xe/MFU-4-Br, for revDSD-PBEP86-D4 in the case of Xe/MFU-4 as well as for all RPA computations, thus in contrast to PBEsol predicting the transition from small to large pore to be lower in energy than the reverse passage from large to small pore. Geometry optimizations at corresponding RPA or double-hybrid level, reducing the remaining basis-set incompleteness error or considering a sophisticated description of the exact exchange contribution for RPA could be remedies expected to overcome these artifacts but are beyond the scope of current work. All barrier energies refer to a temperature of 0 K and a loading of one guest atom, suggesting that the activation energies can change due to the dynamic effects at finite temperature and higher gas loads.