S5 . On Mg(1011), the dimer and trimer processes within a channel (d 0 ↔d 1 , t 0 ↔t 1 ) are well reproduced by the force field (Figure ). In contrast to DFT, however, two separated monomers are calculated to be thermodynamically more stable than the dimer in the d 2 ↔d 3 process. A similar trend is calculated for the trimers. In DFT, a linear trimer structure is the most thermodynamically stable; with the force field, a triangular trimer with the third atom at position t 5 is preferred. Nevertheless, all trimer barriers are reproduced in ReaxFF with a maximum deviation of ∆|E a | = 0.11 eV.