The first goal of this work is to determine whether minimal models of the active site (Si-Min in this case) provide a description of the target reaction mechanism that is in quantitative agreement with that provided by large cluster models (Si-12Å). The first process is the adsorption of H As shown in Figure , the adsorption of H 2 O 2 is exothermic across all choices of models, and the adsorption energies vary considerably with the cluster size if BSSE is not corrected: the average difference between the Si-12Å and Si-Min results is 6.5 kcal/mol, and the difference between the Si-12Å and Si-10Å is 3.5 kcal/mol. As expected, the correction of BSSE is proportional to the fragments' size: 8.9, 7.6, and 4.7 Kcal/mol for Si-12Å, Si-10Å and Si-Min, respectively. With BSSE correction, the differences of Si-10Å and Si-Min from Si-12Å are only 2.1 Kcal/mol and 2.3 Kcal/mol, respectively. It is also notable that the standard deviation of the H 2 O 2 adsorption energies is about 3.2 kcal/mol independently of the different models, which indicates conspicuous variability across sites but not across cluster sizes.