where ω i is the angle between the Nb -O i and O i -Si i bonds, and γ 0i is the dihedral angle between the Nb --O and the O i -Si i bonds; these angles are represented graphically in Figure . For γ 0i , we use the + sign if H 2 O 2 is parallel to the first O in the pocket, and the sign when it is parallel to the second O. This is such that a negative value of γ 0i corresponds to a structure with lower steric hindrance as the Si i center is pointing away from the pocket.