[Ifeffit] Re: Novice FEFF question--feel free to post

Matthew Marcus mamarcus at lbl.gov
Tue Oct 10 09:58:54 CDT 2006


As usual, consistency is everything.  It seems to me that one should always compare with a model compound, if possible.
If you do that and do the model and unknown the same way, then you need only take the ratios of S0^2 or N or whatever.
    mam
----- Original Message ----- 
From: "Bruce Ravel" <bravel at anl.gov>
To: "rtappero" <rtappero at udel.edu>; "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Sent: Tuesday, October 10, 2006 6:52 AM
Subject: [Ifeffit] Re: Novice FEFF question--feel free to post


> 
> Hi Ryan,
> 
> I am taking the liberty of CCing this to the Ifeffit mailing list.  
> 
>> I have a novice FEFF question for you guys.
>>
>>
>>
>> Assuming one has a good idea of the SO^2 value for a specific element
>> either from published literature or analysis of a relevant standard
>> reference phase (e.g. 0.85 for Co in Co(OH)2), what is the consensus on
>> fixing the SO^2 value for the FEFF simulation and if one does fix the value
>> in the feff.inp file (@ 0.85 for this thought experiment) would one still
>> fix SO^2 at the same value during the fit (or fix at one)?
> 
> I'll assume that you are famliar with the normal caveats of the
> assumption of the "chemical transferability" of S02 and that you are
> aware that you introduce a source of systematic uncertainty into your
> analysis when you make that assumption.  If that's not clear, please
> ask so that I or someone else on the list can talk about that.
> 
> I can only answer you question by explaining what I prefer.  I cannot
> presume to speak for the consensus -- different practitioners
> reasonably come to their own conclusions.
> 
> I prefer to run feff with S02=1 and all sigma^2 values set to zero.  I
> then prefer to use Ieffit (or Artemis or WinXAS or EXAFSPAK or
> whatever) to change the parameters of the EXAFS equation from their
> nominal values.
> 
> Of course, setting S02 to 0.85 in Feff and then setting it to 1 in
> Ifeffit is equivalent to setting it to 1 in Feff and 0.85 in Ifeffit.
> While multiplication is unambigously commutative, human brains are
> usually notoriously poor at disentangling a sequence of arithmatic.
> 
> If you: (1) set S02 to something other than 1 in Feff, (2) need to
> modify the value when doing analysis in Ifeffit, (3) go on vacation
> for two weeks, and finally (4) return home to write a paper on your
> analysis --- are you going to remember that you need to multiply your
> two values of S02 together when you report on your analysis in the
> paper?  Hmmmm.....
> 
> Another way of looking at this is to remember what Feff (that is,
> Feff6 -- Feff8 adds a few steps) does:
>  1. computes potentials and phases
>  2. determines the path list
>  3. constructs chi(k) for each path in the path list using the
>     potentials and phases from part 1
>  4. sums up all the paths to make the theoretical chi(k)
> 
> If you consider Ifeffit to be a much more interesting version of the
> fourth part of Feff, then it should be clear that the better approach
> is to use Ifeffit to deal with S02.
> 
> HTH,
> B
> 
> 
> 
> -- 
> Bruce Ravel  ---------------------------------------------- bravel at anl.gov
> 
> Molecular Environmental Science Group, Building 203, Room E-165
> MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007
> 
> Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
> Argonne IL 60439, USA                                fax: (1) 630 252 9793
> 
> My homepage:    http://cars9.uchicago.edu/~ravel 
> EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
> 
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