The origin of the core-level shift of the Si 2p level of surface atoms
and that of the (n-1)s level of adsorbate metal atoms has been invest
igated by means of all-electron ab initio Hartree-Fock cluster model w
ave-functions. The calculated initial-state core-level shifts for both
levels is much smaller than the one expected from an ionic bond where
the interacting systems are charged, and seem to indicate that these
results are not compatible with the formation of an ionic bond. Nevert
heless, the main trends are in agreement with the experimental finding
s. However, a careful analysis carried out by means of the CSOV method
permits an understanding of the origin of these small calculated (or
experimental) core-level shifts and allows us to identify the physical
mechanisms responsible for this behavior.