Hi Shelly, Thanks for the suggestions, here's what happened.
Backfourier transform: Do they miss more at low q or high q? Missing at low q/k would suggest separate dE values while missing at high q/k would suggest separate delr values.
It looks to me like they are missing at low q/k. I will try to attach a graph.
Would your model be better described by using an overall expansion/contraction term rather than one delr value? Using one delr means that all the paths move by -0.01 angstroms. Using one expansion term would allow all paths to contract by 0.1% which might be more reasonable. So that the long paths shrink in more than the short paths. To use a expansion term use the special "reff" variable which stands for the "effective r" i.e. the effective half path length.
I did this and it did help my reduced chi, but I only needed one delr. It didnt seem to make a difference having two. Also, don't forget you have to "define" when using equations, I got an error message when I used "set." Current Results: Chi-square = 4121.564440839 Reduced Chi-square = 390.207284340 Guess parameters +/- uncertainties (initial guess): amp = 0.7738890 +/- 0.0616540 e0 = 1.2375470 +/- 0.8909140 ss = 0.0032920 +/- 0.0005050 e0_1 = -5.3762680 +/- 1.4964040 alpha = -0.0044720 +/- 0.0019360 (-0.0100) There are still some negative values, I guess thats ok? Mucho Thanks , Dan