Group VIB oxides are important in a variety of catalytic processes that involve oxidation,dehydrogenation,and isomedzation reactions.The properties of these oxides differ significantly with variation of the metal.Mixed oxides containing two of the components tungsten,molybdenum,or chromium have been tested,to combine the favorable properties of the two component species.We have been interested in developing cluster models for oxide surfaces and catalysts,in which we carry out theoretical calculations to characterize novel oxide cluster species in the gas phase and to obtain possible mechanistic insight into the complicated surface structures and catalytic processes.More recently,we reported a combined PES and DFT study on a series of tri-tungsten oxide clusters,and suggested that W3O9 cluster can be considered as a molecular model for bulk tungsten oxide from an electronic structure and chemical point of view.In the current work,we report a DFT study on the electronic and structural properties of mixed oxide clusters W2MO92-/1-/0 (M=Mo,Cr).Time-dependent DFT calculations are carried out to compute the vertical detachment energies.Molecular orbitals are used to analyze the chemical bonding in the mixed oxide clusters and elucidate their electronic and structural evolution.