The last energy term E specific summarizes energy contributions that were not relevant to our force field, such as hydrogen bonding or the energy correction in conjugated systems. For a detailed description of the energy terms, we refer to the Supporting Information in the The Mg/Mg parameters were trained using the KVIK optimization routine, which was developed in particular for fast and accurate training of metal ReaxFF force fields. After each optimization cycle, all structures are rescaled with the optimal lattice constant. Otherwise, stresses in the unit cell can significantly affect adsorption energy results or diffusion barriers as Stottmeister and Groß showed. The Mg/O parameters, on the other hand, were trained with the classical successive one-parameter parabolic interpolation (SOPPI). In general, every optimizer has strengths and weaknesses with no guarantee of getting to the global minimum on the first try. Instead, the training process involves many consecutive training cycles, in which the weighting of the training data is continuously adjusted.