“…Equation (5.2) does, however, predict correctly a ninety degree phase shift between direct and scattered X-rays, which is not present in the Born approximation, owing to differing treatments of boundary conditions. We note that Darwin's formula for Bragg diffraction from a bulk crystal [18], when integrated across the rocking curve, predicts a stronger l 3 -dependence of scattered intensity on beam energy because it treats a bulk crystal, does not make the projection approximation made above, and treats full rather than partial reflections. In decisions concerning the maximum beam energy for future XFEL designs, this rapid fall-off in scattered intensity with increasing beam energy must be combined with the dependence of the incident XFEL fluence per shot on beam energy (often decreasing), together with cost estimates to determine a cost/benefit ratio for scattering experiments.…”