[Ifeffit] fourier transforms and k grids

[Peter Pfalzer]

Dear Ifeffit Users,

I changed to iFeffit recently and like the improved functionality very much.
During my test runs I experienced some strange behaviour of the fft functions 
when using a non-standard k grid.

I take from the Ifeffit documentation:
- that all the functions performing fourier transforms should yield the same 
results (given the same parameters as input, of course). 
Specifically: I understand that fftf() and feffit() both compute ffts from k to R 
space for the chi data given with the "real=..." and "chi=..." parameters, 
respectively.
- that these functions all use a k grid starting from 0 and having a grid width 
of 0.05.
- that the non-standard k-grid of my data is automatically interpolated to the 
required k grid whenever the "k=..." parameter is set. This is especially true 
for the fftf() function as well as for the feffit() function.

If these assumptions are right, I do not understand the results of the forward 
fft of the data array performed by the feffit() command.
The fft by feffit() looks like the fft by fftf() when the k array is NOT defined.

Is this behaviour correct? Does it affect the fit? Am I making a silly beginners 
mistake? Is this a bug?

Thanks for your help!
Peter

PS: I am using Ifeffit 1.0076 with the G.I.feffit GUI 0.9 on a Win98SE system. 
I have attached a Ifeffit input file that tests these differences.
--------------------------------------------------------------
Peter Pfalzer
Universitaet Augsburg                    Tel: +49-821-598-3215
Lehrstuhl fuer Experimentalphysik II     Fax: +49-821-598-3411
Universitaetsstr. 1
D-86135 Augsburg
Germany                   Peter.Pfalzer at physik.uni-augsburg.de
--------------------------------------------------------------

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