The lack of scaling between E chem of O* and C* arises from differences in how E hyb and E ortho of the two adsor- bates vary with respect to ϵ d . In this section, we describe 1) why noble metals Cu, Ag and Au fall off otherwise linear scaling relationships 2) why scaling of transition metals is better performed across a fixed row. These two observations come directly from computed energies in Section II. We trace the origin of both these effects to differences in ϵ a values of the two adsorbates. 5 < ϵ d < 0 eV for 4d and 5d metals and -1.6 < ϵ d < 0.9 eV for 3d metals), E hyb decreases with ϵ d . Finally, at large, negative ϵ d values (< -3.5 eV for 4d and 5d metals and < -1.9 eV for 3d metals), E hyb → 0, i.e. it saturates completely for O*, while just starting to saturate for C*.