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.Comment: 18 page