Comparison of the ACE-, GAP-and NequIP-predicted two-dimensional PES over the two forming C-C bonds in R1, where all other degrees of freedom are free to relax, reveals that all three potentials are accurate and smooth in the extrapolation regime (Fig. ). While all three potentials are suitable for training this reaction, ACE offers the best overall balance in terms of data efficiency, training time and accuracy of the final potential (Fig. ). Moreover, even when the closest configuration in the training data is 0.3 Å away in the forming bonds, the error is only a couple of kcal mol -1 when AL is initiated at the TS (r 1 = r 2 = 2.30 Å), suggesting that ACE is accurate in the extrapolation regime (Fig. ). Based on these results, in the following sections we focus our discussions on ACE due to its overall efficiency and accuracy.