[Ifeffit] Manuscript comments regarding EXAFS modeling

[Matthew Marcus]

Was there a standard measured that's chemically similar to the unknown? 
  If so, S0^2 could be set by requiring that the CN of the standard be 
what it's known to be.  Often, the dσ² is mostly relevant in terms of 
differences or changes so an error in the absolute value isn't very 
important.  If valence and ligands make such a big difference then it 
would be impossible to measure CNs on samples for which no 
extremely-similar model compounds are available.

Also, it's never been clear to me how doing the fit at three different 
k-powers is better than doing it at just one intermediate value.  The 
idea that it somehow gets you low-k and high-k information in a better 
way seems like an attempt to pull out more independent data points than 
actually exist.  OTOH, proving that you get the same answers, within 
error bars, at different exponents would be a confidence-builder.
	mam

On 8/27/2021 7:40 PM, Peng Liu wrote:
> Dear Ifeffit members,
> 
> I received the following two comments.
> 
> "
> Comment 1: Authors have fixed the amplitude reduction factor (SO2) to a 
> fixed value (0.85). This factor is specific to particular chemical 
> compound and sample preparation and quality (mostly homogeneity), 
> measurement method (e.g. absorption, fluorescence). Authors can find in 
> literature [e.g. Rehr2000] that SO2 for ideal samples (having no other 
> effects) represent multielectron effects, which by definition depend on 
> valence and ligands. Even more, SO2 is correlated with Debye-Waller 
> factor (σ²) and coordination number (CN), so any chosen value will be 
> compensated by CN and σ². As coordination numbers are used as 
> quantitative indicators in discussion and following conclusions. I would 
> request to clarify the selection criteria for SO2 values and advise to 
> revise this approach (i.e. not to fix SO2 as the same value for all 
> samples). I do not expect drastic changes in obtained CN values, but 
> this should be tested.
> 
> Comment 2: As I mentioned previously, coordination number (CN) is 
> correlated with Debye-Waller factor (σ²). My question is: how this 
> correlation is managed (eliminated)? Most probably (in FEFFIT) this is 
> done by using 3 separate values for n (1,2,3), where n is a power in 
> expression chi(k)*(k^n).
> "
> I used Artemis for the calculation. 1) Because S02 and CN are 
> multiplication relations in the EXAFS equation, as we usually do, we 
> fixed S02 to obtain CN for unknown samples. 2) there are outputs 
> regarding the correlation between different fitting parameters from 
> Artemis. Is there a way to manage or eliminate the correlation as the 
> reviewer mentioned using Artemis or Larch?
> 
> If you also could give me some suggestions to answer the comments, that 
> would also be greatly appreciated.
> 
> -- 
> Best Regards,
> 
> Peng Liu
> 
> School of Environmental Studies
> 
> China University of Geosciences, Wuhan, Hubei Province, PR China
> 
> https://scholar.google.com/citations?user=qUtyvokAAAAJ&hl=en 
> <https://scholar.google.com/citations?user=qUtyvokAAAAJ&hl=en>
> 
> http://grzy.cug.edu.cn/049121/zh_CN/index.htm 
> <http://grzy.cug.edu.cn/049121/zh_CN/index.htm>
> 
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