The last comparison is for the activation energy, step S3 in Figure . The average activation energies, computed as in Eq. 5, are shown in Figure . The data for the individual clusters with and without BSSE are reported in Figure of the SI. Although the TS and reactant structures have the same number of particles and basis functions, correcting for BSSE is necessary to obtain reasonable results and good agreement between different model sizes. The broad variability of the activation energy for the larger models (see left-hand plots in Figure ) is due to the reactant geometries where C 2 H 4 has wandered off the active site, providing unreliable energy barriers. BSSE correction ensures that the TS optimized structures match across corresponding clusters of different sizes (as for the C 2 H 4 adsorption optimizations). The average difference between the Si-12Å and Si-Min results in Figure is 1.5 Kcal/mol, due again to the limited mobility of the Si-Min active sites. Although such an energy difference may lead to more pronounced differences in rate constants (see Eq. 1), this is expected to be smaller than other error sources in the simulation, e.g., the intrinsic accuracy of approximate density functionals and finite basis sets. Therefore, the agreement shown in Figure should be considered rather remarkable.