At the second order Douglas-Kroll-Hess (DKH2) level,the B3PW91 functional in conjunction with the relativistic all-electron basis set of valence triple zeta quality plus polarization functions are employed to compute bond lengths,dissociation energies,vertical ionization potentials,and the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps of the small iridium clusters (I,n ≤ 8).These results are compared with the experimental and theoretical data available in the literature.Our results confirm the theoretical predictions made by Feng et al.about the catalytic activities of the Ir4 and Ir6 clusters.From the optimized geometries,DKH2 calculations of static electric mean dipole polarizabilities and polarizability anisotropies are also carried out.It is the first time that the polarizabilities of small iridium clusters have been studied.For n ≤ 4,the mean dipole polarizabilities per atom present an odd-even oscillatory behavior,whereas from Ir5 to Ir8,they decrease with the cluster size increasing.The dependence of the polarizability anisotropy on the structure symmetry of the iridium cluster is verified.