To keep this section compact, we discuss only the performance of the terrace self-diffusion barriers in the main text but refer to the Supporting Information for the additional diffusion processes (Figure , Table for Mg(0001); Figure , Table for Mg(1011)). Overall, the trained force field reproduces the reference DFT activation energy values of 32 of the 37 processes to an accuracy of |∆E a | ≤ 0.2 eV. As a necessary trade-off, the remaining 5 diffusion barriers, which include exchange terrace self-diffusion, dimer formation, or the step-down diffusion process on Mg(0001), show larger deviations. The trained ReaxFF force field describes all hopping terrace self-diffusion processes within 0.1 eV accuracy but struggles to describe the corresponding exchange processes (∆|E a | ≥ 0.3 eV). In an exchange process, the diffusing atom takes the place of a surface atom, which in turn is pushed to its destination. However, since these processes have much higher barriers than the hopping processes, they are of less importance for an accurate description of surface mobility. In general, terrace self-diffusion on Mg(0001) (Figure ) has almost no barrier, which is also the case for diffusion within a channel on Mg(1010)A (Figure ). In contrast, the activation energy for diffusion across a channel is several times higher, which is in the same order of magnitude as diffusion on Mg(1011) (Figure ). ) due to the exponentially decreasing bond order at long bond lengths. However, since the affected structures constitute high energy phases that are not sampled in simulations of MgO at ambient conditions, the impact of this methodological limitation on the intended scope of simulations with the trained force field is deemed negligible. The corresponding indicator, the heat of formation ∆ f E, is reproduced for all structures with a relative error smaller than 10 % as shown in Table . The rocksalt structure is calculated to give the lowest heat of formation with -5.49 eV with the trained force field compared to -5.57 eV in DFT.