Rebonding effects in separation and surface-diffusion barrier energies of an adatom pair

For Al adatom pairs on Al(100), first-principles calculations imply a small separation energy of 0.07 eV, resulting from Al-surface rebonding as adatom-adatom bonds break. Calculated barriers for Al-pair and single-Al diffusion are 0.66 and 0.80 eV. The pair barrier is lower because pairs reside farther above surfaces, where potential corrugation is weaker, and because an Al surmounting a barrier gains energy from its partner, whose bond to the surface strengthens.