We soon realized that it is not possible to analyze the C I and C II structures simultaneously with a single model. Although the C II structures tend to have lower barriers, we have too few in the set ( ) for a meaningful model fitting. We also removed two C I structures where H 2 O 2 H-bonds to a PBS, as these are also too few for the fitting. Thus, we focus the analysis on 41 C I structures. After considering a large number of descriptors, we settled on 6 descriptors that take into account the available volume as well as steric hindrance from the nearby Si centers. The first two parameters are the θ ij and ϕ ij angles defined in Eq. 7. However, since the O -O bond of H 2 O 2 tends to align parallel to the Nb --O group or one of the Nb -O bonds (see σ values in Table of the SI), it is useful the extend the definition of these two angles to include 0 (for O 0 ) as one of the i, j indices in θ ij and ϕ ij . This is represented graphically in Figure . We then define two descriptors for steric hindrance within a pocket: