These two simple examples demonstrate additional important details. First, while some species within the formalism may represent well-defined locations (critical points) on its PES, others will require a qualitative rule to be invoked. Second, each graph edge corresponds to a different elementary reaction coordinate with this associated with (local) bond-stretching vibrational modes that describe each end of the desmotropic reaction coordinate. In many cases, two or more graph edges in sequence comprise a "full reaction coordinate" (e.g., Fig. ). The last point to note is that not all atoms in a chemical system need undergo atom-connection permutations (e.g., as in Fig. ). Further, the full scope of atom-connection permutations need not be applied. For example, the "active" H-atom in both examples in Fig. have the constraint of maintaining at least one bond.