With the validated reference electronic structure method, we followed the intrinsic reaction path from the transition state to the reactant and the enolate as shown in Figure (curve in blue). When we calculated the potential energy with the DFTB3 Hamiltonian on the same structures along the reaction path, significant deviations appeared at the transition state (see the orange curve at Figure ). Moreover, the DFTB3 Hamiltonian presented a local minimum on the potential energy surface close to the transition state instead of a maximum.