Effective three particle forces in polyvalent atoms
  We study effective three-particle interactions between valence electrons, which are induced by the core polarization. Such interactions are enhanced when valence orbitals have strong overlap with the outermost core shell, in particular for the systems with partially filled f-shell. We find that in certain cases the three-particle contributions are large, affecting the order of energy levels, and need to be included in high-precision calculations. 
M. G. Kozlov, M. S. Safronova, S. G. Porsev and I. I. Tupitsyn
physics.atom-ph physics.chem-ph