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It is investigated how magnetic impurities modify the behavior of metallic clusters. Two complementary models are used, an s-d exchange model with a stable magnetic moment and a Hubbard-type Kondo model. The s-d and s-f interactions are modeled by a pointlike potential, as in the Ruderman-Kittel-Kasuya-Yosida approximation, but the perturbed levels are obtained by diagonalizing the interaction matrix rather than using perturbation theory. The spin polarization of the conduction electrons due to the magnetic impurities leads to a reduction of the highest occupied molecular orbital–lowest unoccupied molecular orbital splitting. A particularly interesting case is Pt, which is used in catalysis and whose well-delocalized 5d electrons are easily spin polarized by conduction electrons. Strikingly, the simplest realization of the Kondo effect is reproduced by an unrestricted Hartree-Fock approximation.