the uneven distribution of interstitial carbon atoms. The density of the final a-C model structure is $\rho\sim2.0$ g/cm$^3$ in good agreement with the density of the a-C substrate as given by the manufacturer. The surface layer of the final structure is smooth but has a corrugation of $d = 0.9$ Å over a distance of a few C-C bondlengths (Fig. \[Au-substrate-potentials\]).

We have calculated the Au/a-C substrate potential at several positions roughly along a line through a depression in the surface. Since for the graphite surface the Au on top position is the preferred bonding site, only Au on top sites were considered in the a-C case. High lying C surface atoms retain their graphite character and are predominantly 3-fold coordinated. Accordingly we find the binding energy of an Au atom is still comparable to the binding energy on graphite (case a in Fig. \[Au-substrate-potentials\]). C atoms lying in a depression of the surface have additional bonds to the nearby C atoms below and are mostly fourfold coordinated. The calculated binding energies of an Au atom to C sites lying in such a depression are considerably larger as compared to the graphite surface (cases b,d in Fig. \[Au-substrate-potentials\]). An Au 