Hi Steven,
 
In general you can use as small of a k-range as you want but you loose the number of variable that you can determine and your uncertainty in those variable gets really big too.    If those trade offs are acceptable then there is no problem.  I assume that you want to determine a few variables so Scotts equation is all you need to determine the minimum k-range.  Artemis also lists the number of independent points in the log file.  That number has to be greater than the number of variables you can determine in the fit.  In practice, you need about twice as many independent points as variables to get reasonable uncertainties.
 
But if what you say about your data is true then the metal peak is not real.  In the chi(k) data the signal for each peak starts at k=0 and goes until the sigma2 term damps it out, usually around 8 to 10 inverse angstroms for oxygen, but usually much larger values for atoms with more electrons.  It is physically impossible for the signal of the metal peak to only exist between 6 and 8 inverse angstroms.   I would compare the data set that you are having troubles with to the other data sets that you indicated gave a consistent peak in the FT.  I would bet that the troubled data set has some artifact in it that is distorting the FT.
 
Shelly
-----Original Message-----
From: Steven Sangyun Lim [mailto:steven.lim@yale.edu]
Sent: Tuesday, April 27, 2004 6:31 PM
To: ifeffit@millenia.cars.aps.anl.gov
Subject: [Ifeffit] k range

Thank you all very much for the immediate help.
One more question, may I???
What can the minimum kmax be?
I think it can be about 8 according to Scott.
If I use kmin=2, can it be 7 or 6? The reason why I ask this is that when I plot R space with my sample, which has been reduced and needs to be confirmed if there is any metal cluster, the metal peak shows at kmax=8, but it disappears at kmax=6 or 7. At kmax=10, it is hard to tell because the spectrum is very noisy (it shows split peak). When there are extremely small metal clusters (even at CN=0.6), I can see a distinct metal peak that does not change no matter what k range and weight I use. I wonder any further advice can be suggested. 
Thank you for your help in advance.
Steven