As different Ga sites are created, the resulting oxygen atom of the Si-OH group can be either distant from, or close to, Ga. The possible extremes in active site structure are, in turn, represented by two Ga sites with substantially different Ga -Si-OH distances, and these are used as illustrative cases in the analysis that follows. In one case, the original Si-O bonds are elongated (Figure : a=1.80 Å, b=1.66 Å, c=1.76 Å, and d=1.69 Å). As a result, the addition of hydrogen to the oxygen atom, leading to the cleavage of the bond (a), gives a final Ga-O distance of 4.57 Å after DFT optimization. In the second case, the Si atom used for creating the Ga site is located in a much more constrained framework, reflected by the shorter Si-O bonds (a=1.66 Å, b=1.62 Å, c=1.67 Å, and d=1.64 Å). As a result, the newly formed Si-OH group gives a Ga-O bond length of 2.02 Å. In the first of these cases, the Ga atom covalently binds to three oxygen atoms, which makes the Ga site three-coordinated. In the second case, the additional, short, Ga-Si-OH bond is considered as another coordination. Hence, the two extremes are defined as the three-and fourcoordinated Ga sites (3CN and 4CN).