The polarization can arise from the occupied-virtual orbital mixing within the adsorbate (as polarized by metal surface) or those within the metal cluster (in response to the nuclei and electron densities of the adsorbate). As shown in Figure (a), when located in the hollow site of metal surfaces, a C atom (L, the ligand) is mainly perturbed through its alpha electrons. Specifically, as shown in Figure (a), the 2p x orbital of a C atom on Pt surface loses 0.905 alpha electrons (0.540 e according to Table ) to its 2p z orbital. Meanwhile, the polarization energy within the Pt and Rh metal clusters, which is nearly equally distributed among the alpha and beta electrons, is higher than those for Ag/Au/Cu clusters. This is expected because Pt 30 and Rh 30 , with an average d 9 s 1 and d 8 s 1 electronic configuration for each metal atom (see Table ), respectively, have empty d orbitals to accept electrons (see Figure (b) for the case of C-Pt 30 ), in contrast to the d 10 s 1 Ag/Au/Cu atoms. For a CH molecule on the metal surface, Figure (a) shows much smaller polarization energy for the adsorbate, and slightly weaker polarization of the metal surface.