  We discuss all the characteristics of Yu-Shiba-Rusinov states for clusters of
impurities with classical magnetic moments in a superconducting substrate with
s-wave symmetry. We consider the effect of the multiorbital structure of the
impurities and the effect of the crystal field splitting. We solve the problem
exactly and calculate the subgap Green's function, which has poles at the
energies of the Shiba states and defines the local density of states associated
to their wave functions. For the case of impurities sufficiently separated, we
derive an effective Hamiltonian to describe the hybridization mediated by the
substrate. We analyze the main features of the spectrum and the spectral
density of the subgap excitations for impurities in dimer configurations with
different relative orientations of the magnetic moments. We also illustrate how
the same formalism applies for the solution of a trimer with frustration in the
orientation of the magnetic moments.
