The adsorption of Cl-2 at a low-coordinated oxygen site (edge or corner site) and vacancy site (terrace, edge, corner F, F+, or F2+ center) has been studied by the density functional method, in conjunction with the embedded cluster models. First, we have studied the adsorption Of Cl-2 at the edge and corner oxygen sites and the results show that Cl-2, energetically, is inclined to adsorb at the corner oxygen site. Moreover, similar to the most advantageous adsorption mode for Cl-2 on the MgO (001) perfect surface, the most favorable adsorption occurs when Cl-2 approaches the corner oxygen site along the normal direction. A small amount of electrons are transferred from the substrate to the antibonding orbital of the adsorbate, leading to the Cl-Cl bond strength weakened a little. Regarding Cl-2 adsorption at the oxygen vacancy site (F, F+, or F2+ center), both large adsorption energies and-rather much elongation of the Cl-Cl bond length have been obtained, in particular at the corner oxygen vacancy site, with concurrently large amounts of electrons transferred from the substrate to the antibonding orbital Of Cl-2. It suggests, at the oxygen vacancy site, that Cl-2 prefers to dissociate into Cl subspecies. And the potential energy surface indicates that the dissociation process of molecular Cl-2 to atomic Cl is virtually barrierless. (C) 2004 American Institute of Physics.