Effects of phonon-phonon coupling on low-lying states in neutron-rich Sn   isotopes
  Starting from an effective Skyrme interaction we present a method to take into account the coupling between one- and two-phonon terms in the wave functions of excited states. The approach is a development of a finite rank separable approximation for the quasiparticle RPA calculations proposed in our previous work. The influence of the phonon-phonon coupling on energies and transition probabilities for the low-lying quadrupole and octupole states in the neutron-rich Sn isotopes is studied. 
A. P. Severyukhin, V. V. Voronov and Nguyen Van Giai