for training are for a three-layer slab model of a (4×4) Ru(0001) surface cell, fully covered with hydrogen, i.e., trajectories start with 16 H atoms in 16 fcc sites of the surface -see Figure below. In ref 17, all Ru atoms were kept fixed in their equilibrium position, a reasonable approximation due to the large mass mismatch between H/D and Ru atoms. We had access to more than 300 trajectories, each 8000 steps long with a time step of 0.1 fs. In total, that is over 2 400 000 DFT energies. For the initial training dataset, we sampled every 50th step of 250 trajectories. For the initial validation set, we sampled every 150th step, shifted for 20 steps compared to the training set. The training was performed on energies only using root-mean-square error (RMSE) as a loss function.