Dear FEFF and IFEFFIT users, As Matt notes,

Feff tries to estimate many of the the physical processes identified as So2. It also estimates lambda, but much more crudely: it interpolates a fit to the *figures* from Rahkonen and Krause (1974). Feff definitely does not include the energy resolution of a Si(111) monochromator at 9keV.

One of the biggest problems with the current FEFF calculation is that the mean free path lambda is calculated from the Hedin-Lundqvist electron gas self energy in the plasmon-pole approximation. This often overestimates the inelastic losses (too short mean free paths). The consequence of this is that the effective So2 must often be increased to compensate. For this reason, it is not unusual to see fits of So2 that that can exceed unity. Fortunately improvements are in sight. Luke Campbell's thesis focused in part on calculations of So2, and he was able to explain the So2 for Cu (about 0.9) to within a few percent. Also efforts are being made to improve the self energy. However, at present the theoretical So2 is typically 0.9 +/- 0.1. This of course, does not take into account possible errors in the value of the edge jump and corrections for experimental resolution which can affect fits to So2. John Rehr