The adsorption and dissociation of different coverage of water molecules on Fe(100)surface has been calculated using DFT density functional methods.The most stable adsorption configuration of H2O is at atop site nearly paralleling to the iron surface.With the coverage of water increases,the preference structures for the water oligomers form a buckled structure,where the donor and acceptor are at different distances from the metal surface.For the aggregation adsorption of water molecules,the most stable structure is a pentagon-based structure.The most stable geometries are in the balance between water-water and water-metal bonding.The direct H2O monomer dissociation is favored both kinetically and thermodynamically.The preferred pathway for the dissociation of nH2O proceeds one after another.For the dissociation of H2O on the nO pre-coveraged surfaces,the energies of the system are gradually increased with the increasing of the surface O atoms.In the dissociation process of the first O-H bond of H2O,H2O can dissociate more easily to 2OH +(n-1)O via an activation energy less than 0.32 eV.The transition state of the dissociation for the first O-H bond is close to the initial state of nO + H2O.Then the 2OH dissociate in the order of proton acceptor OH and proton donor OH.Through the studies for the reaction of nO + H2O,one can see that H2O can dissociate on the O pre-covered surface as long as there exist the free adsorption sites for the best dissociation of OH.That is to say,once there is no free adsorption site for the best pathway of dissociation for OH,it is not preferred to proceed continually.