Influence of 2hω excitations on the low-energy spectra of nuclei

The angular momentum projected deformed particle-hole model (projected Tamm-Dancoff) is applied to the positive parity states of the nuclei 20Ne and 24Mg. For the single particle states a large basis containing three major shells is used. The effects of a truncation of this basis to the 1s-0d shell only are studied, and the possibility of describing these effects by a perturbation expansion is investigated. It turns our that a perturbation expansion in terms of self-consistent projected Tamm-Dancoff states yields a very good agreement already in lowest order, while the usual renormalization of the interaction in terms of spherical one particle-one hole states fails to reproduce the result obtained in the large model space. The reliability of the projected Tamm-Dancoff model is confirmed by a comparison to an exact shell model calculation