A detailed comparison of the activation energy results calculated with ReaxFF and DFT is provided in Table . We previously showed that Mg dimer formation on Mg(0001) occurs barrier-free and that its cleavage is associated with a barrier that is several times larger (Figure and S4b). The trained ReaxFF force field reproduces this trend only qualitatively: the barriers for dimer formation are shallow (0.11 eV and 0.12 eV) but so are the reverse barriers (0.28 eV and 0.29 eV). A similar overestimation occurs for the diffusion of a Mg atom towards a [1120]-directed step-edge (0.22 eV vs. 0.03 eV; Figure ) but diffusion along the step-edge is accurately reproduced (both 0.22 eV; Figure and).