Finally, it was shown in Part 3 that the trends among the different functionals are retained when moving from periodic calculations, in which the positions of the guest molecules were fully optimised, to single-point calculations using frozen cluster models cut out from PBE-D3 optimised structures. Given the rather large variations in the equilibrium positions of the adsorbates observed in Part 2, at least for some cases, this relative "robustness" of the trends might appear rather surprising. First, it can be inferred that it may be sufficient to pre-optimise the structures with one functional prior to calculations of the adsorption energies using a larger number of functionals in order to get a first overview. In this way, the set of approaches could be narrowed down to the most promising ones, avoiding computationally demanding optimisations with dc-XC functionals exhibiting systematic errors. Second, the analysis confirms that clusters can serve as useful model systems to benchmark DFT approaches against higher-level calculations, for which periodic calculations may be computationally very expensive or even completely unfeasible.