Description of collective 0+ states in the Tamm-Dancoff approximation

The Tamm-Dancoff approximation is applied to the correlated FBCS ground state. This procedure proves very successful for medium and heavy nuclei both superconducting and non-superconducting. The accuracy of the Tamm-Dancoff approximation is equivalent to forth order boson methods. Calculations have been performed for the nickel isotopes, for Sn¹¹⁴ and for the lead isotopes. The connection with the broken-pair approximation is established. The influence of a nucleus withN?2 nucleons on the spectrum of a nucleus withN particles is exhibited.