For the Pd flat surfaces, the most favorable sites for H adsorption are the hollow (4-fold) and the hollow (3-fold) fcc sites for Pd(100)  and Pd(111)  faces, respectively. These are representative of the large particle size domain. For the large Pd clusters, we can select N4(8,8,8,8) adsorption sites for Pd(100), starting from m = 4, and N3fcc(9,9,9) adsorption site for Pd(111), starting from m = 6. In the case of a smaller Pd cuboctahedron cluster (m = 2, 13 atoms), two stable sites for H adsorption were found by Watari et al. . One is the hollow (4-fold) N4(5,5,5,5) inside the square face, and the other one is the hollow (3-fold) hexagonal close packing (hcp) N3hcp(5,5,5) of the triangular face. It has to be mentioned that these sites exist only for small particle sizes, since for m = 2 most of the surface atoms display a coordination number of 5. For intermediate particle size, several 4-fold adsorption sites are coexisting on the square face, which are a combination of coordination number 5 (corners), 7 (edges), and 8 (faces). This leads to two additional possibilities, which are N4(5,7,7,8) resulting from an edge atom creation, starting from m = 3, and N4(7,7,8,8) resulting from an additional face atom creation, starting from m = 4. In the same way, additional 3-fold hcp adsorption sites on a triangular face have to be taken into consideration as the crystallite size is increasing. These are N3hcp(5,7,7), starting from m = 3 and N3hcp(7,7,9), starting from m = 4. As mentioned above, 3-fold hcp sites are the most favoured for small crystallite sizes, whereas 3-fold fcc are favoured for large sizes. In this way, the additional two 3-fold hcp sites permit the transition between small and large crystallites.