Almerindo and Pliego studied the hydroylsis of formamide with ab initio calculations and PCM . 1-4 explicit water molecules were considered, and both stepwise and concerted mechanisms were investigated. For the stepwise mechanism, the activation barrier increased by 4.6 kcal/mol with one water molecule (Figure ) and 11.0 kcal/mol with two water molecules (Figure ). Adding water molecules beyond two made the system entropically unfavorable. For the concerted mechanism, the solvation only increased the barrier by 6.4 kcal/mol with two water molecules (Figure ), indicating that the transition state is more stabilized by the solvent than in the stepwise mechanism. Additionally, the level of theory used for the calculations had a large effect on the barriers. The discrepancy caused by the level of theory