Dear Wojciech, I presume you mean the "amp_x" value, this topic was discussed in January 07 on this list and Scott Calvin and Kelly Shelly gave useful hints (The subject was "is there a physical meaning to a negative S02") It all boils down to the question what happens if you multiply sin(x) with -1. So it seems that the model you use in your FEFF calculation is off by a half an oscillation. regards, Edmund Welter Wojciech Starosta wrote:

Sorry if it is FAQ. But I spent some time on fitting without success and have not found answer to this.

Does anybody could explain me if negative value of amplitude has any sense. I can get beautiful plot after fit, but unfortunately the message appear concerning negative value of amplitude (amplitudes). How to avoid it. Any written material will be very helpful. I know that I can define amplitude as squared value of new variable. But sometimes it does not work. Ifeffit stops with message that value of this variable cannot be determined

With the best regards

Wojtek Starosta Institute of Nuclear Chemistry and Technology 03-195 Warsaw, Dorodna 16 Institute of Nuclear Chemistry and Technology

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