Periodic DFT calculations were performed with the plane-wave based Vienna ab initio simulation package (VASP). The core electrons were described using the projector augmented wave (PAW) method of Blöchl as implemented in VASP. Exchange correlation effects were described within the generalized gradient approximation (GGA) of Perdew, Burke, and Enzerhof (PBE). Additionally, the Bayesian error estimation functional with van der Waals correlation (BEEF-vdW) was applied to determine the standard deviation of the investigated quantities. A plane-wave cutoff energy of 400 eV was employed after detailed convergence studies (SI Figure ), where Mg 2s electrons were described as valence electrons. Following the scheme of Monkhorst and Pack, a k-point mesh density of at least 0.14 Å -1 was applied for all total energy calculations. Thermal smearing of one-electron states was allowed using the Gaussian smearing method (σ = 0.1 eV) to determine the partial occupancies of each orbital and acquire faster convergence with respect to the number of kpoints. The electronic self-consistent field (SCF) was considered converged when the total energy difference was less than 10 -5 eV, and the norms of all the forces were smaller than 10 -3 eV•Å -1 . Surface energies were calculated on symmetrical (1×1) cells with a minimum slab thickness of 30 Å and a minimum vacuum region of 20 Å (SI Table ). A single atom in the middle of the slab was fixed to avoid net translations. Adsorption energies, dimer interaction, diffusion barriers, and frequencies were generally calculated on converged slabs with 6 surface layers, where the two uttermost layers were fixed to emulate bulk properties. If it became necessary, vicinal slabs were used. The exact surface characteristics are summarised in SI Table . The minimum energy path (MEP) was determined with the climbing image nudged elastic band (CI-NEB) method as implemented in the transition state tools for VASP (VTST) with generally 7 images between the initial states separated by a spring constant of 5.0 eV•Å -2 . As an initial guess for the MEP the image-dependent pair potential (IDPP) was applied. To confirm the transition state and to determine rate constants, we calculated vibrational frequencies with the dynamical matrix method from the VTST package.