Using ab initio methods. we theoretically investigate the adsorption of one and two Pt dimers oil Cu(001) Treating the interaction of adsorbates oil surfaces as a local phenomenon, a representation of the substrate by a large cluster of at least 62 Cu atoms allows one to treat the electronic structures of both systems, that is, the adsorbate and the surface, on equal footing. Theoretical results concerning the adsorbate energetics, structure, and density of states are presented We find that the Pt atoms of the adsorbates prefer to locate on top of the hollow sites of the Cu lattice. A symmetry-adapted-cluster expansion configuration-interaction method is used to calculate the excited states and the optical absorption spectra of the systems. The most intense peaks of the absorption spectra are around 1 eV and result from excited states of B-2 symmetry Last but not least, we identify surface-mediated interdimer interactions.