to assess the anharmonic scale factor λ Anh for each transition structure conformer. This factor represents the ratio between the anharmonic ZPE and the harmonic ZPE; its product with the standard harmonic scale factor λ H = 0.984 93 for M08-HX/MG3S yields the overall SRP scale factor. In HDCPT2 calculations, optimized geometries and second-order derivatives were obtained by M08-HX/MG3S, while higher-order numerical derivatives were from MPW1K 148 /MG3S. This choice is driven by the sensitivity of numerical derivatives of (hybrid-)meta-GGAs to DFT integration grids; hence, a (hybrid-)GGA is preferable for 3rd and 4th derivatives in HDCPT2 calculations.