In this section, we discuss the surface diffusion of chemisorbed intermediates, assuming that valence-saturated parts of alkyl chains can only physisorb on the surface due to dispersion attraction. Since dispersion interactions are quite weak for rather small C1 and C2 species and vary very smoothly with the atomic positions, they are known to have a minor effect on diffusion barriers. For example, the adsorption free energy of CH4 on Cu(111) and Pd(111) is 0.03 and ~0.00 eV, respectively, indicating that the enthalpy of physisorption has roughly the same magnitude as the free energy contribution of the entropic stabilization of the gas phase molecules. Although Pd is a more strongly bunding metal than Cu, it still exhibits facile diffusion of most adsorbates. Figure (∆𝐺 ‡ = 0.24 eV), which is a crucial step for the subsequent hydrogenation reactions on Cu. Other less significant reaction steps may also occur at or near edge sites.