In subsequent iterations, both approaches successfully sample the PES, as shown in the right panels of Fig. absolute error (MAE) in the energy for the trained MLPs is 2.66 meV atom -1 for downhill AL and 2.12 meV atom -1 for WTMetaD-IB AL. Both values are within chemical accuracy (1 kcal, equating to 43 meV and 7.16 meV atom -1 for this system). The predicted energy errors for each configuration in the IRC are depicted in the subplot of Fig. . The individual predicted energies for both MLPs are below 1 kcal mol -1 as well, except for one data point predicted by MLP trained using the downhill AL with an error of 7.16 meV atom -1 (1 kcal mol -1 ). Interestingly, while the TS optimized by the ground-truth method was not provided in WTMetaD-IB AL, the MLP trained using this method still reached a higher accuracy for the TS with an error of 2.37 meV atom -1 in comparison to 2.63 meV atom -1 by downhill sampling. This shows the applicability of the WTMetaD AL in sampling the region close to TS and the ability of the resulting MLP to predict the energy of the TS. We evaluated the quality of the trained MLPs using an independent test set obtained from a 3.2 ps umbrella sampling (US) simulation at the ground-truth level of theory (US/AIMD).