The H 2 O + HNCO products, carbamate and imidic acid, can be formed via a concerted reaction mechanism in the presence of one or two catalyst waters. In these cases, the catalyst waters form six or eight atom ring transition states, respectively including either the N --C or C --O bonds. The IRCs from these processes connect to pre and post reactive van der Waals complexes which are reasonably strong relative to the separated molecules (between 5-10 kcal mol -1 for the single water catalyzed case at the ωB97x-D3/def2-QZVPPD level of theory). These complexes are excluded from our analysis as they are decreasingly relevant at higher temperatures, and preliminary computations indicated that substituents have far less influence on the energy of the van der Waals complexes than on the transition state energies.