Noniterative two-πexchange in the nuclear medium

An effective $\mathrm{NN}$ interaction is constructed in the framework of noncovariant perturbation theory, containing a one-boson exchange part and explicitly the noniterative $2\pi$ exchange terms involving nucleon intermediate states. The one-boson exchange parameters are adjusted to fit $\mathrm{NN}$-scattering phase shifts and the data of the deuteron. The resulting interaction is used in Brueckner-Hartree-Fock calculations for nuclear matter and finite nuclei. The contributions of the so-called stretched-box and crossed-box terms to the binding energy are investigated. Special attention is paid to the many-body effects which quench the contribution of the noniterative $2\pi$ exchange terms in the nuclear medium.