For the energies of the stationary points, the HEAT-345Q results are included in Figure for comparison. The PES is in good agreement with the CCSD(T)-F12a/AVTZ calculations, with a maximum deviation of only 0.003 eV at the transition state. All energies are relative to the CH3NC if not specified hereafter and the PES energies do not include the zero-point energy. The barrier on the PES is 1.727 eV. With harmonic ZPE correction, the barrier becomes 1.651 eV, much close to the experimental result, 1.665 eV, and the value calculated by HEAT-345Q with ZPE correction of 1.659 eV. The harmonic frequencies of the stationary points are listed in Table with results from available refs for comparison. The PIP-NN PES accurately reproduces the ab initio calculation results with a largest deviation of only 18 cm -1 and in excellent agreement with the CCSD(T)/ANO2 results for the largest deviation of 19 cm -1 . For the CH3CN molecule, we compared the frequencies with the spectroscopic experimental data, which includes anharmonic effect, and found that the two results are in reasonable agreement. In addition, anharmonic parameters were calculated by the vibrational perturbation theory with the PIP-NN PES interfaced to the Gaussian program. The results are shown in Supplementary Material in the low triangular form and compared with previous CCSD(T)/ANO1 results. The RMSEs for the anharmonic rotational constants are 2.15 cm⁻¹ for TS, 1.72 cm⁻¹ for CH₃CN, and 3.65 cm⁻¹ for CH₃NC, with corresponding mean absolute errors (MAEs) of 1.56 cm⁻¹, 1.57 cm⁻¹, 1.72 cm⁻¹, respectively, demonstrating its accuracy in studying the anharmonic effects by the PES but with greatly reduced calculation cost.