Elster and Weppner [Phys. Rev. C 57, 189 (1998)], conclude from comparisons of their results with proton elastic scattering data that an optical potential formed from a model based upon the impulse approximation to the scattering is inappropriate below 200 MeV. That is certainly the case with their model, however folded models using $\mathrm{NN}\hspace{0.5em}g$ matrices have been shown to be appropriate. They claim that higher order terms in the spectator expansion are the required corrections and that a Faddeev calculation is needed. We show that the less than satisfactory comparisons of their calculated results with data are due to their use of free $\mathrm{NN}\hspace{0.5em}t$ matrices as the transition operators. Comparison of predictions obtained from the (complex, nonlocal) optical model potentials we find by folding appropriate medium modified $\mathrm{NN}$ interactions with data from the scattering of 65 and 200 MeV protons from ${}^{40}\mathrm{Ca}$ illustrate this.

• Received 9 January 1998

DOI:https://doi.org/10.1103/PhysRevC.59.1813