As mentioned in the introduction, the scaling relationship was rationalized by Nørskov and coworkers with a d-band model that states the differences in adsorption energies across the metals is mostly due to interactions with the d states. As such, it invoked a separation of contributions to adsorption energy from sp and d electrons of transition metals. The sp-electron contribution was suggested to remain nearly constant from metal to metal, whereas the d-electron contribution varies more substantially and underlies the observed linear scaling relationship. To explore this in the context of adsorption on the cluster model of metal surfaces, a further separation of the polarization and charge transfer energies into their sp and d orbital contributions is carried out using the procedure described in Section 2.B, with the results shown in Figure .