Initial training of an ACE potential from the TS (R2 TS1 ) generated ≈ 450 configurations using standard AL with sampling in the reactant and product regions of two of the products (R2 P1 and R2 P2 ). However, even when propagating MLP-MD trajectories at 500 K, the Cope product (R2 P3 ) was not observed in the training data. Only when the TS that leads directly to R2 P3 was included as an AL starting configuration was the training data sufficiently complete. This second TS was 4.4 kcal mol -1 higher in energy than the TS that leads to R2 P1 and R2 P2 . This result highlights the importance of considering relevant points in the PES that may not be obtained using automated sampling methods when training MLPs. For this reaction, ACE training required ≈ 2 days of AL.