All first principles calculations to obtain the training set for the parameterization of the Mg/O parameters were performed with the periodic plane-wave DFT code Vienna ab initio simulation package (VASP). The description of the core electrons was carried out using the projector augmented wave (PAW) method where the Mg 2s and O 2s4p electrons were considered as valence electrons. The exchange and correlation energy was described using the Perdew, Burke, and Enzerhof (PBE) functional following the generalized gradient approximation (GGA) approach. After a detailed convergence study, we chose the cutoff energy to be 500 eV (Figure ) and the k -points sampling using a gamma-centered mesh (due to the hexagonal symmetry of hcp Mg) with a minimum grid density of 0.14 Å-1 (Figure ). We also ensured that the number of k -points in one direction was always rounded up to the next odd integer. To provide consistency with the Mg/Mg training set, we used Gaussian smearing with a width of 0.1 eV for the one-electron states as we did for metallic magnesium. The convergence criterion for the electronic self-consistent field (SCF) was set to a maximum energy difference of 1 × 10 -5 eV between two consecutive minimization cycles, and the norms of all forces had to be less than 1 × 10 -3 eV Å-1 . Surface energy results were calculated using (1 × 1) cells with a minimum thickness of 20 Å and an equally large vacuum region. Oxygen adsorption energy results and diffusion barriers were calculated on (4 × 4) Mg(0001), (4 × 2)