We demonstrated that small cluster models, which include the metal center and the first layer of Si centers, provide values of ∆E ads of the reactant and ∆E a that are close to those obtained with large cluster models (Si-12Å in our case), see Figure . However, the large models must be pre-optimized at DFT level to reduce procedural disorder. The errors on the averages are around 1-2 Kcal/mol for adsorption energies and around 1.5 Kcal/mol for energy barriers; these differences are likely due to the fact that the Si-Min models are necessarily more rigid than the Si-12Å models and cannot relax as much as the large clusters during the reaction. Although these errors may seem large for the evaluation of reaction rates, they are definitely smaller than the expected accuracy of approximate density functionals and basis sets in practical applications. The varia-tion across sites is also similar between large and small clusters, which indicates that the variability of activity across sites is captured by the small models. An important aspect is that BSSE must be corrected during the geometry optimization of both reactants and TS structures in order to obtain comparable results across models. This is particularly important for the Si-12Å and Si-10Å models because they are larger, but also because C 2 H 4 may wander away from the active site during the optimization due to the low binding energy. The good performance obtained with the minimalistic site models is very important from a computational point of view, as the calculations on the large models are resource and time intensive.