With on-the-fly training, each MC step is first evaluated by a cNNP. If the uncertainty of the predicted energy is above the specified threshold, the step is further evaluated by the reference method, in this case PBE-D3, and the configuration is added to the training set. The cNNP is constantly retrained as structures are added to the training set to ensure reliable uncertainties and avoid adding redundant structures. For the MC simulations described here we use a rigid molecule moveset consisting of: molecular rotations and translations as well as unit cell lengths, angles, and volume changes. Conformational moves can be added to explore intramolecular perturbations to the crystal structure. Further details of the simulations are provided in the supporting information.