  The effects of hydrogen impurities in the bulk and on the surface of aluminum
are theoretically investigated. Within the framework of density functional
theory, we have obtained the dependence on H concentration of the stacking
fault energy, the cleavage energy, the Al/H surface energy and the Al/H/Al
interface formation energy. The results indicate a strong dependence of the
slip energy barrier in the $[\bar 211]$ direction the cleavage energy in the
[111] direction and the Al/H/Al interface formation energy, on H concentration
and on tension. The dependence of the Al/H surface energy on H coverage is less
pronounced, while the optimal H coverage is $\leq 0.25$ monolayer. The
calculated activation energy for diffusion between high symmetry sites in the
bulk and on the surface is practically the same, 0.167 eV. From these results,
we draw conclusions about the possible effect of H impurities on mechanical
properties, and in particular on their role in embrittlement of Al.
