  We investigate the electron transfer from a dissociatively adsorbed H2
molecule to a Au(111) surface using the first-principles methods. A fractional
electron transfers from a molecule to a substrate, and potential energy
increases during the process. The initial energy increase coincides with that
of the isolated, separated, and positively charged H2 molecule calculated by
the real-space density functional method. The barrier formation is due to the
destabilization of the molecule induced by the electron transfer. The
electronegativity difference between the adsorbate and the substrate determines
the direction of the electron transfer.
