Ground-state correlations beyond RPA

An approach is proposed which includes ground-state correlations beyond RPA. This is achieved by using a renormalized quasiboson approximation with reference to the correlated ground state of the system, rather than to the uncorrelated Hartree-Fock one, and by expressing the particle-particle and hole-hole pair operators in terms of the particle-hole ones. The set of the so-obtained nonlinear equations for the one-phonon energy and amplitudes are first solved within the Lipkin-Meshkov-Glick model and the results are compared with those of previous approaches as well as with the exact ones. Applications to realistic nuclei, starting from fully self-consistent HF + RPA calculations with a Skyrme force are also presented. ((orig.))