Potential energy surfaces for chemical reactions can be classified as attractive or repulsive by comparing the extensions of the bond lengths in the activated complex relative to those of the reactants and products. For a reaction of type A + B—C → A—B + C, the bond length extension for the newly formed A—B bond is defined as R* AB = R AB − R AB, where R AB is the A—B bond length in the transition state and R AB in the product molecule. Similarly for the bond which is broken in the reaction, R* BC = R BC − R BC, where R BC refers to the reactant molecule.