First-principles calculations based on the density functional theory of the adsorption and migration energies of Mg atom and MgO molecule on a flat MgO (0 0 1) surface are reported and discussed. The isolated neutral Mg adatom binds quite loosely to the surface (E-ads similar or equal to 0.46 eV) and has to overcome a barrier of about 0.30 eV in between two stable 0 adsorption sites. The MgO molecule forms iono-covalent bonds with the surface, with an adsorption energy of 2,5 0,1 eV, At variance with the isolated Mg adatom. the surface diffusion of the MgO molecule may imply a sequence of elementary rotation step around its ends. The computed barriers for molecule, 90: rotation around its 0 and Mg atoms are 0.35 and 0.46 eV, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.