where Φ X is the PF per volume of the transition state (X = ‡), reactant 1 (X = R1), or reactant 2 (X = R2). Notice that, throughout this whole article, we are taking the zero of energy for all PFs as the potential energy at the equilibrium geometry of the infinitely separated bimolecular reactants; thus, the barrier height is implicitly contained in Φ ‡ .