In certain cases, the determined enthalpies of the catalyzed transition states are negative relative to the separated products, but are still referred to as "barrier heights". This should be understood as a consequence of selecting the separated reactants as a reference, rather than selecting the water trimer and HNCO molecule as a reference, for example. Figure presents the ∆H 0K barrier heights for each process relative to the separated reactants. In all cases, additional water molecules significantly lower the enthalpy of the transition state, with the first catalyst water having the largest effect and lowering the transition state barrier by at least 20 kcal mol -1 . This is unsurprising and NBO E results indicate significant delocalization between the additional water molecules, lowering the energy of the transition state ring structures. Catalytic water molecules have a greater impact on the TSO barriers, which results in the TSO3 and TSN3 barriers being reasonably competitive in some cases with a difference of less than 4 kcal mol -1 across all substituents. Figure by at least 15 kcal mol -1 ), regardless of substituent . However, the energetic benefit of a second catalyst water is negligible as it is overcome by the loss of translational entropy. This is in qualitative agreement with the work of Wei and coworkers who demonstrate convergence in the Gibbs energy of the reaction barriers with more than two catalyst waters.