Noniterative isobar diagrams and their effect inNNscattering

The fourth-order noniterative diagrams involving nucleon-isobar intermediate states and including $\pi$ exchange are calculated in momentum space in the framework of noncovariant perturbation theory. A potential model is presented which includes, apart from these diagrams, suitable one-boson-exchange terms and iterative isobar diagrams involving $N\Delta$- and $\Delta \Delta$-intermediate states, considering $\pi$ and $\rho$ exchange for the transition potentials. Finally, noniterative two-$\pi$-exchange diagrams involving two-nucleon intermediate states are also taken into account. Such a model is able to describe $\mathrm{NN}$ scattering phase shifts satisfactorily. The role of the different parts of the potential model in $\mathrm{NN}$ scattering is studied.