Finally, the activation barriers calculated using Equation ( ) for the diffusion process between the oct adsorption site and its subsurface equivalent sub-oct on Mg(0001) were part of the training set (Figure ). According to our DFT calculations, an oxygen atom must overcome a barrier of 0.68 eV to diffuse from an oct position to a subsurface site and 0.81 eV to diffuse back. The corresponding barriers in ReaxFF are 0.71 eV and 0.73 eV and thus agree well. We calculated additional oxygen diffusion barriers for Mg(0001), Mg(1010)A, and Mg(1011) to validate the obtained parameters, which were not part of the training set. The performance of these diffusion processes is discussed in the Supporting Information (Figures S7, S8, and S9; Tables , and S5).