resulted in a more accurate potential (Fig. , MAD = 2.1 kcal mol -1 ). A modest improvement was obtained upon hyperparameter optimisation, at moderate computational cost (≈ 500 configurations required for R1). Specifically, reducing the regularisation, increasing the quality of the radial basis, and doubling the number of atomic environments considered in the training all improved the GAP (SI §S2). However, attempts to improve GAPs by training a component-wise potential separated over covalent bonds were unsuccessful (SI §S3.3).