The improved tangent nudged elastic band (IT-NEB) algorithm was used for finding the minimum energy path, which gives the accurate energy and configuration for the saddle point. By measuring the energy difference between the saddle point and the local and global minima, it is possible to determine the activation energy. The initial (x=0.0) and the final (x=1.0) configurations for the NEB method were constructed by placing the guest atom in the middle of the large pore. The algorithm was then requested to interpolate 64 beads in between, with the middle (x=0.5) configuration being the guest atom in the middle of the small pore (Fig. ). The spring constant for the NEB beads was set as 0.05 in atomic units, with a convergence threshold of 1x10 -3 hartree/bohr. As for the cell optimizations, the PBEsol functional with D3-correction to account for dispersion was used together with a DZVP-MOLOPT-SR basis set. Core electrons are approximated using Goedecker-Teter-Hutter pseudopotentials. Single-Point Energy Computations The performance of generalized gradient approximation (GGA)based functionals was assessed by increasing the accuracy to meta-GGA, hybrid and double-hybrid density functionals (DHDFs). Performing NEB calculations even at hybrid functional level is very costly for the systems under investigation. Therefore, only single-point energy computations of PBEsol optimized structures along the minimum energy path were performed to assess the hierarchy of the density functionals PBE0, rSCAN, ωB97X-2 and revDSD-PBEP86-D4. For highest accuracy, we performed RPA computations. For the assessment, basis-set size was increased from double-zeta to triple-zeta quality choosing a TZV2P basis for the PBE0 and rSCAN single-point energy computations and a ccGRB-T basis for the ωB97X-2, revDSD-PBEP86-D4 and RPA computations, respectively. RPA basis-set convergence was extrapolated based on results for ccGRB-D and ccGRB-T basis sets to get an estimate for the RPA basis-set incompleteness error. Furthermore, computations for the DHDFs ωB97X-2, revDSD-PBEP86-D4 and for RPA were performed with a threshold of 10 -9 a.u. for the Schwartz integral screening and a wave function grid of 150 Ry for the correlation contribution. Cutoffs for exchange were checked to be converged by increasing the wave function cutoff from 150 Ry to 200 Ry leading to negligible differences in the total energies. Convergence for the cutoff radius for the exchange contribution was checked for values between 8 to 10 Angstrom, resulting in consistent and thus converged energies.