Core polarization of the effective two-nucleon interaction for different descriptions of the core

The core polarization of the effective two-nucleon interaction is studied for different descriptions of the core degrees of freedom, viz. perturbation theory, the Tamm-Dancoff approximation (TDA) and the random phase approximation (RPA). The core states were calculated both for a bare particle-hole interaction and for a renormalized one, in which the self-screening of the exchange term is the most important component. For the bare particle-hole force TDA and RPA give low-lying, collective 0<SUP>+</SUP> and 2<SUP>+</SUP> core phonons and lead to significant renormalization of the two-nucleon interaction. Screening of the particle-hole interaction serves to push up these core phonons and reduce the core-polarization effect on the two-nucleon interaction. The core polarization obtained for the screened RPA is close to the one obtained for the bare TDA