The　adsorption　of　Cl2　Na　monolayers　supported　on　the　MgO(001)　surface　has　been　studied　by　the　density　functional　method　using　cluster　models　embedded　in　a　large　array　of　point　charges　(PCs).　The　value　of　PCs　was　determined　by　charge　self-consistent　technique.　The　results　indicate　that　Na-promoted　MgO(001)　surface　is　an　efficient　catalyst　toward　Cl2　adsorptive　decomposition.　Besides,　it　was　found　that　the　role　of　the　MgO(001)　surface　is　not　passive,　which　is　different　from　CO　adsorption　on　MgO(001)　surface　supported　Na　metal　monolayers.　The　analysis　of　band　and　projected　density　of　states　indicates　that　the　electron　transfer　from　the　surface　Mg　3s　valence　orbital　and　Na　3s　valence　orbital　to　the　anti-bonding　σ*　orbital　of　Cl2　is　the　source　of　the　Cl2　bond　weakening.　This　is　also　different　from　the　CO　adsorption　on　MgO(001)　surface　supported　Na　metal　monolayers,　where　only　the　electrons　from　the　Na　valence　orbital　are　transferred　to　the　anti-bonding　π*　orbital　of　adsorbed　CO.　Our　study　suggests　that　the　essence　of　catalysis　is　different　for　CO　and　Cl2　adsorption　on　Na　metal　monolayers　supported　an　MgO(001)　surface.　©　2003　Elsevier　Science　Ltd.　All　rights　reserved.