[Ifeffit] R-factor
Alison L Costello
acostell at unm.edu
Wed Jul 6 10:11:47 CDT 2005
Bruce,
I've looked, and my R-factors for the back-transformed
space are not necessarily twice the value of the R-factors
for the k-space fits. In fact, they are often quite
close. For example, just recently I had a k-space
R-factor of 0.102 and a q-space R-factor of 0.113. Now, I
realize those numbers are very close, but I'm afraid if I
try to publish this, then I will get criticism for the
q-space R-factors being larger. If I can explain it, then
maybe it won't be a problem.
I do see what you're saying about the complex vs. real
function, and that definately makes sense, but they don't
differ by a factor of 2. Hmmm.
alison
On Wed, 6 Jul 2005 09:43:03 -0500
Bruce Ravel <bravel at anl.gov> wrote:
> On Tuesday 05 July 2005 18:39, Alison Costello wrote:
>> I have a question regarding the calculation of the
>>R-factor. I am
>> currently using SIXPACK to process my protein EXAFS
>>data, and I fit the
>> data in both k-space and in q-space (back-transformed
>>k-space). I
>> typically will use k-ranges = [1-13.6] and for
>>multiple-scattering fits,
>> will use an R-range of 0.1 - 4.5. Sometimes, the
>>R-factor for the q-space
>> fit is greater than the R-factor for the k-space fit,
>>which should not be
>> true, as the back-transformed space filters out noise.
>> I am confused as to
>> why this occurs, and am wondering if the R-factor for
>>the back-transformed
>> space is calculated differently than for k-space fits.
>
> Alison,
>
> I would expect the R factor to be bigger in k space due
>to the high frequency
> portions of the data that get filtered out of the q
>space data. That is what
> happened in the one example I just tried.
>
> What are some example values of the R-factors? Do they
>differ by about a
> factor of 2?
>
> One thing that occurs to me is that chi(q) is a complex
>function while chi(k)
> is a real function. In Ifeffit those two functions
>should have the same
> number of points, but the R factor in q is computed
>using both parts of the
> complex function. Thus it would seem reasonable if they
>differed by about 2
> and there were no significant Fourier components in the
>data beyond your rmax.
>
> B
>
> --
> Bruce Ravel -----------------------------------
>bravel at anl.gov -or-
> ravel at phys.washington.edu
> Environmental Research Division, Building 203, Room
>E-165
> Argonne National Laboratory
> phone: (1) 630 252 5033
> Argonne IL 60439, USA
> fax: (1) 630 252 9793
>
> My homepage: http://feff.phys.washington.edu/~ravel
> EXAFS software:
>http://feff.phys.washington.edu/~ravel/software/exafs/
>
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