Our first objective was to make the attractive region of the C-H RPF's more repulsive, specifically the intervals between 2.1-2.7 Å and 2.9-3.2 Å shown in Figure . For each tested configuration of the LCHFs, labeled as CH_X (with X=1 to 9; see Figure and Table S2 for more details), we recalculated the potential energy along the reaction path for the modified DFTB3 Hamiltonian. As shown in Figure , the addition of harmonic functions eliminates the local minimum at the transition state, approximating the activation barrier to the reference DFT method.