Another key goal of this work is to emphasize that multiple transition state (TS) configurations can be found for a single active site. An example is shown in Figure , where several TSs are found that vary in the relative position of the reactive species around the metal or in the relative orientation. Each TS structure has a corresponding reactant minimum well on the potential energy surface (PES) and these are also different from each other. All of these structures describe the same reactive event, but they can have drastically different adsorption energies for the reactive species and, more importantly, different reaction barriers. In this section, we try to quantify how much adsorption and activation energies are affected by the local structure of different active sites. Since we established in section 3.1 that the Si-Min clusters are reasonable models of the Si-12Å clusters, we use the former for the following analysis. As discussed in section 2, we consider eleven Si-Min models (all were carved out from pre-optimized Si-12Å clusters): five sites with three-member oxygens ring (3M), five sites with isolated Si-O groups (ISO) and one site with two-member oxygen ring (2M), see Figure . In order to distinguish where each TS structure is located for each site, we need to define an unambiguous notation to identify different areas around the metal center. We denote the oxo O as O 0 and we define the angle