  In this work, we have studied theoretically the effects of gold adsorption on
the Al(001) surface, using {\it ab initio} pseudo-potential method in the
framework of the density functional theory. Having found the hollow sites at
the Al(001) surface as the most preferred adsorption sites, we have
investigated the effects of the Au adsorption with different coverages
($\Theta$=0.11, 0.25, 0.50, 0.75, 1.00 ML) on the geometry, adsorption energy,
surface dipole moment, and the work-function of the Al(001) surface. The
results show that, even though the work-function of the Al substrate increases
with the Au coverage, the surface dipole moment decreases with the changes in
coverage from $\Theta=0.11$ ML to $\Theta=0.25$ ML. We have explained this
behavior by analyzing the electronic and ionic charge distributions.
Furthermore, by studying the diffusion of Au atoms in to the substrate, we have
shown that at room temperature the diffusion rate of Au atoms in to the
substrate is negligible but, increasing the temperature to about 200$^\circ$ C
the Au atoms significantly diffuse in to the substrate, in agreement with the
experiment.
