We determine the geometry of the ground state and of several low energy metastable configurations of oxygen adsorbed on Mg(0001) for coverage $1/16\le \Theta \le 1$. In agreement with experiments, we find that oxygen is adsorbed below the Mg surface, forming ionic islands commensurate with the metal lattice. Molecular dynamics simulations show how ${\mathrm{O}}_2/\mathrm{Mg}\left(0001\right)$ spontaneously dissociates and incorporates below the Mg top layer. The collision and the progression towards the most stable configuration involve complex, many-atom processes.

• Received 14 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4433