The oxygen atom diffusion barriers on Mg(1011) are in agreement with the DFT reference activation energy (Table ). E.g., the diffusion path from the hlw position via the tet-2 site (0.59 eV vs. 0.54 eV) to the tet-3 site (0.16 eV vs. 0.05 eV), as well as the reverse direction (0.25 eV vs. 0.19 eV for tet-3 to tet-2 and 0.51 eV vs. 0.49 eV for tet-2 to hlw), is accurately reproduced by the force field (cf. Figure and). However, differences emerge due to the overestimated thermodynamic stability of the oct position calculated by the force field. The diffusion of an oxygen atom from the tet-3 site (0.45 eV) or from the fcc to the oct site (0.13 eV) is highly preferred over its counterpart when calculations are performed with the force field (0.62 eV and 0.82 eV). In DFT, this sequence is reversed, and low barriers are associated with diffusion from the oct to the tet-3 site (0.16 eV) or the fcc position (0.11 eV). Equally of interest is the diffusion pathway for an oxygen atom at the S12 hlw position coordinated by five Mg atoms towards the fcc or oct sites. While in the DFT, the oxygen atom significantly prefers to diffuse to the fcc position (0.66 eV), the trained force field predicts diffusion to the oct position (0.66 eV) (cf. Figure and). Oxygen was counted as being in the atmosphere when two oxygen atoms were closer than 1.5 Å, but both were more than 2.45 Å away from a Mg bulk atom; they were considered adsorbed if two oxygen atoms were closer than 1.5 Å and one was closer than 2.45 Å from a Mg bulk atom or bound if one oxygen atom was closer than 2.45 Å to a Mg bulk atom but farther than 2.45 Å from the nearest oxygen atom.