Figure shows the minimum energy path (MEP), with the energy changes along the reaction path, for the PES-2023 and the ab initio input data at the CCSD(T)-F12/augcc-pVTZ//MP2 level. PES-2023 reasonably simulates the entrance channel, which is very flat and difficult to calculate, with the largest differences in the exit channel, about 2.5 kcal mol -1 . However, considering all points used in the fitting process, the overall rootmean-square error (RMSE) is 1.7 kcal mol -1 , which seems reasonable given the high number of degrees of freedom in this polyatomic system. A more stringent test was performed using geometries not used in the fitting and again comparing the energy obtained with PES-2023 and CCSD(T)-F12/aug-cc-pVTZ. We begin by calculating a scan of points, where the breaking C-H' and forming H'-CN bond lengths vary in the range 1.0-3.0 Å. These geometries correspond to energies in the range between +25 and -25 kcal mol -1 , i.e. a range of ~50 kcal mol -1 . On this scan, 10 randomly scattered geometries were selected for comparison. In this case, using points outside the points used in the fitting, the RMSE is slightly larger, 2.4 kcal mol -1 . This result seems reasonable given the wide range of energies used and proves the accurate and limitations of the new surface. Figure (upper panel) presents the bi-dimensional contour plot using the PES-2023, where the breaking C-H' and formed H'-CN bonds are represented in the range 0.9-3.0 Å, the remaining coordinates being fixed in the saddle point. This contour plot covers all dynamically relevant zones for the reaction, from the entrance channel with the weak reactant complex to the very exothermic product channel with the stabilized product complex. In addition, it shows the smooth, continuous and differentiable behaviour of PES-2023.