Hi, Matt and Carlo and other EXAFS experts

Attached please find a fpj data file. The basic system is silica + PO4 ligand on surfaces, which was used for adsorption of La. We collected La L3-edge EAXFS data and did this fitting. I used two models of La binding as monodentate and bidentate. I saw the negative ss parameter. I saw this in some of my other EXAFS data fitting samples as well. I have two questions for help:

1. What might cause the negative ss parameters and how I may improve this, is this caused by incorrect structure models
2. If we can do some “stress” test to make sure the fitting is valid and meaningful.

Please you can help look at and play this data set and direct me on how to do this correctly, thank you so much.

Dien Li

Savannah River National Laboratory