Nonlocal-exchange effects in elastic photon scattering from low-Z elements

We investigate the importance of using nonlocal versus local exchange in describing the elastic scattering of photons from low-Z elements in the range 2≤Z≤18. Correction factors are given that can be used to account for nonlocal exchange effects starting from a cross section evaluated using local exchange, as, for example, in local independent-particle-approximation S-matrix calculations. The correction factors are derived using the form-factor approximation, and are therefore only applicable in the above-threshold regime where the beyond-form-factor-approximation anomalous scattering amplitudes are comparatively small. However, it is seen that even well above threshold, the nonlocal correction can be as large as ∼10% or more for low-Z elements, as has recently been demonstrated experimentally for the case of neon