  We present a density functional study of the structural and electronic
properties of small Cu_n (n=1,4) aggregates on defect-free MgO(100). The
calculations employ a slab geometry with periodic boundary conditions,
supercells with up to 76 atoms, and include full relaxation of the surface
layer and of all adsorbed atoms. The preferred adsorption site for a single Cu
adatom is on top of an oxygen atom. The adsorption energy and Cu-O distance are
E_S-A = 0.99 eV and d_S-A = 2.04 Angstroems using the Perdew-Wang gradient
corrected exchange correlation functional. The saddle point for surface
diffusion is at the "hollow" site, with a diffusion barrier of around 0.45 eV.
For the adsorbed copper dimer, two geometries, one parallel and one
perpendicular to the surface, are very close in energy. For the adsorbed Cu_3,
a linear configuration is preferred to the triangular geometry. As for the
tetramer, the most stable adsorbed geometry for Cu_4 is a rhombus. The
adsorption energy per Cu atom decreases with increasing the size of the
cluster, while the Cu-Cu cohesive energy increases, rapidly becoming more
important than the adsorption energy.

Materials Science