Hi Paul,

I was curious to tap the collective wisdom of those on the list regarding the "true meaning" of S02, the passive electron reduction factor, at least with regards to fitting experimental data to feff calculated paths.

It may be (?) that the "meaning" of S02 from fits may differ from the "true meaning" from a theoretical perspective. Luke Campbell looked at this question in his thesis work [see "Interference between Extrinsic and Intrinsic Losses in XAFS," L. Campbell, L. Hedin, J. J. Rehr, and W. Bardyszewski, Phys. Rev. B 65, 064107 (2002)], and found that, as had been anticipated, S02 is only weakly energy and path dependent. The work also found that the intrinsic loss estimate for S02, i.e. the many body overlap integral ||**2 which is typically 0.7 is too small, due to interference between intrinsic and extrinsic losses. Instead S02 is more like 0.9. This also means that there is about 10% of the spectral weight in multi-electron excitations, which can be modeled roughly as an additional broadening term (and hence a shorter mean free path). The current estimates of S02 in FEFF are not very satisfactory. FEFF will attempt to calculate this overlap integral (i.e., intrinsic losses only) if one uses the card S02 0 in feff.inp. But, the Hedin-Lundqvist self energy has too much loss, which can only be corrected for by increasing the value of S02. Fortunately these errors partly compensate each other. Doing better requires better many body coding. Luke Campbell's codes are not yet implemented yet - but we hope to add them in one of the next releases. We are also working on better self-energies. Stay tuned on this. Thus at present it's still better to fit but not to let S02 vary too much. In my view S02 might be best modelled as a constant about 0.9 +/- 0.1 plus an additional broadening term (that is by allowing the variable ei to vary by about an eV or so in feffit), i.e., S02= 0.9 exp(-2R/lambda(ei) I seem to recall that Matt Newville has tested such fits. A further problem with fitting S02 in practice is that fits to experimental data depend critically on the edge jump. To minimize this, I would recommend adjusting the near edge structure to theoretical calculations (e.g., from FEFF8) to fix the edge jump as well as possible, rather than simply fitting it.

standards. How about the expected temperature dependence of S02? I do

I think the temperature dependence should be very weak. Mosly the path dependence is in a correction to the mean free path. I'll be curious to see what others say about S02 especially fixing the edge jump or fitting ei rather than S02 or both ei and S02. Cheers, John Rehr