To illustrate the improvement of the potential we first ran simple downhill Monte Carlo simulations on the 300 oxalic acid structures (Figure ). These simulations, which only accept Monte Carlo moves that decrease the energy, are relatively localized, constrained effectively to the initial energy basin. Nevertheless, using the initial cNNP trained on CSP minima we find that only 9 CSP structures remain stable after 1500 MC steps. The other 291 trajectories were terminated early due to high uncertainties in energy predictions in excess of 50 kJ mol -1 . By contrast, performing the same simulations with the on-the-fly trained cNNP 299 trajectories remain stable.