On the energy-shell contributions of the three-particle~-~ three-hole   excitations
  The response functions for the extended second and third random phase approximation are compared. A second order perturbation calculation shows that the first-order amplitude for the direct $3p3h$ excitation from the ground state cancels with those that are engendered by the $1p1h$-$3p3h$ coupling. As a consequence nonvanishing $3p3h$ effects to the $1p1h$ response involve off energy shell renormalization only. On shell $3p3h$ processes are absent. 
A. Mariano, F. Krmpoti\'c and A.F.R. de Toledo Piza