However, consistently higher activation energies are observed for the cluster model. For example, for the ethylene insertion step (species 2 to species 3 ), the cluster and periodic catalyst site models give activation energies of 1.8 and 1.5 eV, respectively. The difference may be attributed to long-range order in the silica support, which may cause a lower activation energy, but is absent in the cluster model. Moreover, the models are evaluated with distinct functionals due to their differing availability in the reference molecular and periodic quantum chemistry packages being used. Nevertheless, the two approaches predict similar relative barriers for all of the TS under each elementary step type. The mean difference between activation energies for type I versus type II reactions are 0.8 and 1.0 eV, calculated by the cluster model and the periodic model, respectively.