Electronic polarity of nanoclusters: quantum and many-body effects
  Interesting electrical polarity in nanoclusters usually requires the polarizability to exceed the value R^3 of the classical sphere of radius R. We clarify how this occurs naturally in single electron quantum systems, and relate it to the giant polarizability of Na_14F_13, and to spontaneous dipole formation on niobium clusters. Many-body effects generally reduce the polarizability through screening. The usual RPA treatment retrieves the classical answer, but it significantly overestimates screening in few-electron systems. The system of two electrons on the surface of a sphere is solved numerically, to account for the Coulomb repulsion. At high densities, numerical results agree with RPA model with properly subtracted self-interaction effects. At low densities, the system performs quantum oscillations around the classical ground state. We calculate the lowest anharmonic correction to the polarizability, which also agrees well with numerical evaluation of the polarizability. 
A.V.Shytov, P.B.Allen