Collective and non-collective multiphonon states in even-even spherical nuclei

The multiphonon method previously developed for even-even spherical nuclei used pairs of fermions as building blocks. The present extension is based on collective and/or non-collective phonons. Calculation of matrix elements of one- and two-body operators between multiphonon states can be reduced to the determination of overlap matrix elements of states containing phonons and some pseudo-phonons General recursion formulae for the evaluation of such elements are given and allow for numerical calculations in a space spanned by a reasonably large number (n = 5-6) of realistic phonons.