According to DFT, an oxygen atom diffuses on Mg(0001) (Table ) in a zigzag route first from the fcc site by overcoming a barrier of 0.41 eV to the thermodynamically most stable tet-1 position (Figure ). From there, the oxygen atom moves to the oct position via the meta-stable tet-3 site with an activation energy of 0.72 eV. Finally, the oxygen atom overcomes 0.37 eV to reach the tet-2 position. In calculations with the trained force field, an oxygen atom adsorbed at the fcc position diffuses vertically down to the oct position overcoming a barrier of 0.20 eV (Figure ), or from the hcp to the tet-1 site overcoming 0.17 eV (Figure ), thereby underestimating the barrier by 50 % compared to DFT. The diffusion barriers within the surface are the same magnitude as the DFT barriers, with 0.54 eV from the tet-1 to the oct and 0.81 eV from the oct to the tet-2 site, respectively.