In the case of Ir, the most favorable sites for H adsorption are the bridge and the top sites for Ir(100)  and Ir(111)  faces, respectively, corresponding to the N2(8,8) and N1(9) adsorption sites, both starting from m = 4. Davis et al. calculated that the most favorable H adsorption sites for 38 atom truncated octahedron Ir cluster are the bridge edge sites , indicating that the equivalent position N2(5,7) and N2edge(7,7) adsorption sites have to be taken into account for small cuboctahedron clusters. Moreover, two types of adsorption sites have been suggested on the basis of DFT calculation for tetrahedron Ir4 cluster. These additional adsorption sites are top (corresponding to the N1(5) adsorption site for cuboctahedron clusters) and bridge position at Ir–Ir bonds (corresponding to N2(5,5) adsorption sites for cuboctahedron clusters) . Starting from m = 3, an additional bridge site N2(7,8) appears and has to be considered as another adsorption site.