  From microscopic point-dipole model calculations of the screening of the
Coulomb interaction in non-polar systems by polarizable atoms, it is known that
screening strongly depends on dimensionality. For example, in one dimensional
systems the short range interaction is screened, while the long range
interaction is anti-screened. This anti-screening is also observed in some zero
dimensional structures, i.e. molecular systems. By means of ab-initio
calculations in conjunction with the random-phase approximation (RPA) within
the FLAPW method we study screening of the Coulomb interaction in FexOy
clusters. For completeness these results are compared with their bulk
counterpart magnetite. It appears that the onsite Coulomb interaction is very
well screened both in the clusters and bulk. On the other hand for the
intersite Coulomb interaction the important observation is made that it is
almost contant throughout the clusters, while for the bulk it is almost
completely screened. More precisely and interestingly, in the clusters
anti-screening is observed by means of ab-initio calculations.
