with the other O centers directly bound to the Nb. We call O α the oxygen with the largest χ i , and the others are defined sequentially in clockwise order when looking along the Nb = O bond from the top. We then define three pockets P of accessible volume for the reactants around the metal center; P a is defined as the pocket between O α and O β , and the others are again defined sequentially in clockwise order. These rules are summarized visually in Figure .