The non-linear ML fitting does not provide a clear fundamental picture of why a certain site is more active than another one. However, in this particular case there is one descriptor that has distinct influence on the reactivity: the ϕ ij angle. A direct plot of ∆E a vs ϕ ij in Figure clearly indicates that a larger available volume in the pocket leads to lower energy barrier, at least for the C I structures. We can infer that this is because the reactants have closer access to the metal center. However, more data points, especially on C II structures, are needed to formulate more definitive conclusions.