Configuration interaction study of single and double dipole plasmon   excitations in Na_8
  We carry out a microscopic analysis of the ground and excited states of the Na_8 metal cluster within the jellium model. We perform a series of configuration interaction calculations on a Hartree-Fock basis and construct eigenstates of the Hamiltonian which carry up to 4-particle 4-hole components. Based on the analysis of the dipole transition strengths, we single out those states which can be interpreted as the collective dipole plasmon and its double excitations. These modes are found to possess a high degree of harmonicity, deviations from the harmonic limit remaining, however, of the order of 10%. 
F. Catara, D. Gambacurta, M. Grasso, and M. Sambataro