First principles calculations of the potential energy surface for H-2
dissociation on the (0001) surface of Mg are presented, based on a pla
ne-wave pseudopotential formalism within local density functional theo
ry. It is shown that there is no molecularly chemisorbed precursor sta
te, in contrast to earlier, jellium-based calculations. The lowest dis
sociation barrier (0.37 eV per molecule) is found for dissociation abo
ut a bridge site into neighbouring centre sites. A considerable surfac
e corrugation is found, both for atomic adsorption and for the propert
ies of the transition state. Charge density difference maps, showing t
he charge transfer between the metal and the molecule, support the pic
ture that dissociation occurs through occupation of anti-bonding state
s of the molecule.