Hi Matt and all I sent this and an attachment before. I hope you can help figure out why I often get negative ss parameter from fitting? For this specific example, if I fit the CN, it yields CN of ~1, the monodendate structure is reasonable, but I got negative ss. However, if I fixed CN at 3, the ss is reasonable, but it needs to bind with three different PO4 on the surfaces, this is interesting but I am uncertain. Thank you so much for your any advice. Dien Li Savannah River National Laboratory -----Original Message----- From: Ifeffit On Behalf Of ifeffit-request@millenia.cars.aps.anl.gov Sent: Friday, September 9, 2022 12:32 PM To: ifeffit@millenia.cars.aps.anl.gov Subject: [EXTERNAL] Ifeffit Digest, Vol 235, Issue 3 Send Ifeffit mailing list submissions to ifeffit@millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit or, via email, send a message with subject or body 'help' to ifeffit-request@millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-owner@millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Help for EXAFS fitting (Dien.Li@srnl.doe.gov) ---------------------------------------------------------------------- Message: 1 Date: Fri, 9 Sep 2022 16:31:38 +0000 From: "Dien.Li@srnl.doe.gov" To: "ifeffit@millenia.cars.aps.anl.gov" Subject: [Ifeffit] Help for EXAFS fitting Message-ID: Content-Type: text/plain; charset="us-ascii" 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