Water and HNCO can react via two different concerted mechanisms, with the water O-H bond breaking across the N --C double bond to form carbamate or across the C --O double bond to form imidic acid (Figure ). Both mechanisms proceed through transition states (TS1 and TS2, respectively) that form four-membered rings between the two reactants. The predicted TS1 barrier (38.5 kcal mol -1 ) is much lower than the TS2 barrier (47.5 kcal mol -1 ). This is qualitatively in agreement with previous research but we predict TS1 to be K Gibbs free energy surface with the loss of translational entropy leading to an average increase of relative energies by about 9 kcal mol -1 for all stationary points and confirms our assumptions concerning the van der Waal complexes.