Role of noniterativeπexchange inNNscattering

The fourth-order noniterative diagrams involving nucleon intermediate states and including $\pi$ exchange are calculated in momentum space, in the framework of noncovariant perturbation theory. The effect of these so-called crossed-box and stretched-box diagrams on $\mathrm{NN}$-scattering phase shifts is studied by adding these diagrams to a one-boson-exchange potential, likewise derived from noncovariant perturbation theory, and solving a corresponding scattering equation.