The first step is to compute reaction energies and eliminate reactions with reaction energies larger than a threshold (here 30 kcal/mol) since the reaction energy provides a lower bound on the barrier height. The products for several of the reactions consist of more than one molecule (or fragment) and many of the molecules are identical since atom mapping does not affect the reaction energy. Thus, to reduce the number of calculations, we identify all unique fragments and perform PM3 calculations on the fragments and calculate the reaction energy as: