The next process is the adsorption of C 2 H 4 , step S2 in Figure . The average adsorption energies, computed as in Eq. and Si-Min. The wide gap between Si-12Å and other models is due to the contribution of certain sites, as shown in Figure of the SI. For example, for site 3 ∆E ads (C 2 H 4 ) is underestimated by 26.1 Kcal/mol with Si-Min compared to the Si-12Å value. This is because the optimized structures are rather different, as shown in Figure of the SI. In the large cluster, C 2 H 4 wanders away from the active site during the optimization to find a better binding site; on the other hand, the small size of the Si-Min model forces the reactant to remain close to the active site. When BSSE is corrected, as in the right-hand side plots in Figure , the average adsorption energies with all models sizes are about 1 Kcal/mol from each other. Interestingly, performing the geometry optimization with BSSE correction provides minima that are consistent across model sizes and with C 2 H 4 bound to the active site. This is consistent with previous results on systems with weak interactions. Because C 2 H 4 is weakly bound to the active site, there is some variability in the relative orientation across corresponding sites in models of different sizes, but the energy differences are small and they will not influence the activation barrier.