LAUSR

Abstract By means of the van der Waals density functional and quasi-harmonic approximation, we predict that the adsorption potential energies of CD4 on various fcc metal (1 1 1) surfaces are shallower than those of CH4, whereas the equilibrium distances from the surface are larger. This isotope effect originates from the softening of the C H bond pointing toward the surface, leading to the significant lowering of its vibrational frequency and hence the large zero-point energy difference between CH4 and CD4. The C H bond softening is attributed to the hybridization of the lowest unoccupied molecular orbital with the substrate state.