The H 2 O 2 adsorption energies of the individual sites are shown in Figures S1 of the SI. There is some variability in the H 2 O 2 orientation between the Si-Min and Si-12Å models, due to H-bonding interactions with the silica surface that are possible in the larger clusters but not in the smaller ones and to the fact that the Si-Min models are not allowed to relax beyond the active site (i.e., the Si and F centers are frozen). When the Si-Min and Si-12Å optimized structures overlap well (as in clusters 1, 2, 3, and 6) the difference in adsorption energy is less than 2 Kcal/mol. When the H 2 O 2 orientation differs (as in clusters 4, 5, and 7), ∆E ads (H 2 O 2 ) can vary by up to 6 Kcal/mol. Despite these differences, H 2 O 2 binds in the same location in the Si-Min and Si-12Å clusters, and the differences in energy are similar to those between the Si-10Å and Si-12Å clusters even though the latter two models provide almost perfect overlap of the optimized structures.