The second key point of this work is to show that the same site can host multiple TS structures, see Figure , all corresponding to the same reaction event but with significantly different barriers. Using 11 cluster models, we found 59 TS structures with activation energies varying from 7 to 26 Kcal/mol, and there is no guarantee that we found all possible TS structures. This is an often overlooked issue, for instance, in microkinetic models where each active site is associated with a single reaction event (i.e., with one set of energy parameters). The evaluation of the reaction rates can be completely wrong if the energy parameters in the model from DFT simulations refer to only one of the possible TS structures in a site. Taking the average of the energy values for a single site is also not a good strategy, because the effective rate is driven by the most active sites and not the average site. In this study, we found that the TS structures could be divided in two categories: one where the active site structure is essentially unchanged (C I ) and one where it is changed (C II ). In the C I structures, the reaction occurs in one of the pockets of the distorted tetrahedral structure of the active site, which gives easier access to the metal center. In the C II category, the Nb --O group has a lowfrequency wagging mode (≤ 100 cm -1 ) so that the oxo group can be pushed away by the reactants to increase the available space near the metal center, and the reaction does not need to occur in one of the pockets. The C II reac-tions have lower barriers than the C I reactions, but ∆E tot ads may or may not be favorable depending on the site. The energy diagrams for representative sites in Figures show that the behavior is similar for structures within the same pocket, but there is still enough variability across TS structures depending on the relative orientation of the molecules with respect to the active site geometry.