Dear Scott,
I have learned the Hamilton test. However, this method seems not suitable to
distinguish our results because the Nvar are same for the two fittings (3 when
S02 was fixed). So the b value should be zero and the calculator on the website
http://www.danielsoper.com/statcalc/calculator.aspx?id=37 can not carry on.
In addition, I am not sure the value x. Is it the ratio of R-factors between
two fits? I calculated the results using a=2.18 (Nidp and variables (Nvar) were
7.35 and 3), b=0.1, and x=0.818 (the ratio of two R-factors) and got the
Regularized lower incomplete beta function of 0.0807. Does it mean
something?
Cheers,
Shaofeng
--
Shaofeng Wang, Ph.D of Geochemistry
Environmental Molecular Science Group
Institute of Applied Ecology, Chinese Academy of Sciences
Shenyang, 110016, China
wangshaof...@iae.ac.cn
www.iae.cas.cn
From: Scott Calvin
Sent: Thursday, January 05, 2017 11:50 AM
To: XAFS Analysis using Ifeffit
Cc: Shaofeng Wang
Subject: Fwd: about sigma2 for exafs fitting
Shaofeng has given me permission to repost her question here on the ifeffit
mailing list. It is quoted below my response.
Dear Shaofeng,
As Bruce and I said before, a sigma2 of 0.0007 A^2 is not impossible, although
it indicates less disorder than is typically present. Your attached table does
seem to show some improvement by using the model from the hydrogen-containing
structure as compared to the arsenate. A more rigorous test for statistical
improvement can be conducted using the Hamilton test (you mentioned you’ve
consulted XAFS for Everyone; full details of the Hamilton test are given
there).
It’s also encouraging that the uncertainties on your sigma2 determinations
using the hydrogen-containing model are quite small; it appears that the fit is
not getting confused by correlations even though it’s fitting both coordination
number and sigma2, as that would generally also cause high uncertainties in the
correlated parameters.
Is such a stiff sigma2 reasonable in this case? I have no idea. I just don’t
know enough about this particular system; perhaps someone else on the list
does. Oh, and one other question—was the data collected at room temperature? If
it were collected at cryogenic temperatures, that would tend to reduce thermal
disorder and thus lower sigma2’s.
Even if no one on the list has insight in to this particular system, anyone
have good published examples of room-temperature systems with sigma2’s < 0.001?
It might help Shaofeng with her referee…
Best,
Scott Calvin
Lehman College of the City University of New York
P. S. I certainly hope no reviewer is using the “typical values” I provide for
parameters in XAFS for Everyone as rigid criteria for rejecting results! It is
most certainly not the way I use them in the book. Some systems actually are
atypical!
Begin forwarded message:
From: Shaofeng Wang
Subject: about sigma2 for exafs fitting
Date: January 4, 2017 at 8:31:33 PM CST
To:
Reply-To: Shaofeng Wang
Dear Dr. Calvin,
I am a research from China. I know you are an expert on XAFS analysis. So I
write this letter to you for some xafs analysisi problem.
Recently, I am attempting to study the incorporation of arsenate into the
barite structure. To investigate the species of arsenate in barite, I fitted
the exafs data using Ba3(AsO4)2 and a dft optimized structure with HAsO4
incorporated in barite supercell (configuration C2), respectively and got some
results. Please see the attached table. Smaller reduced chi2 and R-factor were
obtained by using configuration C2 as the initial model. Does this mean the
species of arsenate in barite is more likely to be HAsO42- instead of AsO43- ?
We also have other evidences including XANES and vibrational spectroscopy to
support this conclusion. However, using C2 as the initial model we obtained
samll sigma2 (~0.0007). This seems too small and out of the normal range (0.002
每 0.03) as you mentioned in your publications (XAFS for everyone). So, my
question is if our data are reasonable. If yes, could you provid some
references to support?
By the way, I asked simialr question on the ifeffit forum and you gave me
some answer. However, the reviewer was not convinced and he insited on that
sigma2 must be in the range of 0.002-0.03. How can I response this question?
Any help is very appreciated.
Best regards,
Shaofeng
--
Shaofeng Wang, Ph.D of Geochemistry
Environmental Molecular Science Group
Institute of Applied Ecology, Chinese Academy of Sciences
Shenyang, 110016, China
wangshaof...@iae.ac.cn
www.iae.cas.cn
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