The largest difference found corresponds to the geometric configuration of the reactant complex (see insert in Figure ). So, while PES-2023 locates a linear C-H'-CN approach, the CCSD(T)/cc-pVTZ level locates a "quasi" cyclic structure. In this last case, we should emphasize that we had many problems of convergence in the optimization process (passing from minimum to saddle point configurations in some iterations of the process), probably due to the use of numerical second energy derivatives at this ab initio level.