Dear Rana, Delta E0 is a shift in the Energy origin of k, that is where k=0. For the experimental data, this was chosen empirically (and so somewhat arbitrarily) from the measured data. For the FEFF calculation, it was chosen at the calculated absorption threshold (lowest available electronic level), but is known to not be perfect at this, especially in Feff6. Though E0 is meant to be the absorption threshold, it is more important that the FEFF calculation and measured data agree. Because of this, Delta E0 is best left as an adjustable parameter in the analysis. It is generally found that a single Delta E0 parameter is needed *per measured data set and per FEFF calculation*, because measured data can be shifted in energy and because each calculation can have a different definition of E0. It is possible to read the FEFF outputs and try to figure out how different these energies are and so set (not allow to float) the different E0 values. I believe this can work well, but it is not automated, and so not often used. In addition, usually one uses a different Delta E0 for each data set, but well-aligned data that re-use a FEFF calculation (say, data taken on the same system at different external conditions) can often get by with sharing a single Delta E0 parameter. S02 is meant to express the incomplete overlap of the "passive electrons" in the absorbing atom. Removing the deep core electron can allow the other (L, M, N, O) electrons to relax slightly from their ground state. That relaxation reduces the overlap of the "initial" and "final" states of the transition that gives rise to XAFS, and so reduces the full effect. FEFF6 does not do a very good job of estimating this effect. In practice, S02 can also mask other "amplitude issues", some theoretical (the inelastic losses that fold into the mean-free-path are not perfect) and some experimental (it is easier to mis-measure the amplitude than the phase of an oscillation). For XAFS, these experimental problems all tend to spoil the amplitude of the meaurement more than the phase: pin-holes in the sample, slight misalignment of the sample, over-absorption for fluorescence measurements, energy bandwidth of the incident beam and harmonic content of the incident beam. The rule of thumb is that it is OK to use the same S02 for all samples of a particular absorbing atom. Many would add "measured at the same beamline" to that rule, so that effects of energy bandwidth and harmonic content are compensated. Cheers, --Matt