[Ifeffit] help: can I use ifeffit to predict chi?

Matthew Marcus mamarcus at lbl.gov
Fri Nov 2 13:20:07 CDT 2007


I have sent Chen some data on magnetite and maghemite.
    mam
----- Original Message ----- 
From: "Tai-Yen Chen" <tychen at mail.chem.tamu.edu>
To: <bravel at bnl.gov>; "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Sent: Friday, November 02, 2007 10:53 AM
Subject: Re: [Ifeffit] help: can I use ifeffit to predict chi?


> 
> Hi, Bruce
>   Thanks for the valuable information. I have tried the 
> sum of pathes method to do the estimation.
> But it seems like not to accurtate compared to the data I 
> found in the paper.
> So, I want to go back to the the proper fit fo find out 
> poper sigma^2 and eo.
> But I don't have any numeric chi data for gamma-Fe2O3. Do 
> you have any idea where I may get these
> information? Thanks for all the help.
> Best Regard!!
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> On Thu, 1 Nov 2007 10:38:35 -0400
>  Bruce Ravel <bravel at bnl.gov> wrote:
>> 
>> On Tuesday 30 October 2007 21:21:59 Tai-Yen Chen wrote:
>>> hi,
>>>    Can anyone tell me whether I can use ifeffit to do 
>>>this
>>> job?
>>>
>>> I want to know the % change in chi when the molecule 
>>>have
>>> 3% change in radius?
>>> All I have is just the crystal struture of gamma-Fe2O3,
>>> which means I can only get the Feff.
>>> How can I introduce 3% change in redius so that I can 
>>>get
>>> the difference of chi?
>>> Can anyone help me? Tanks a lot.
>> 
>> Hi TaiYen,
>> 
>> Since you are asking about using Ifeffit, I presume that 
>>you are
>> interested in changes in the EXAFS spectrum rather than 
>>the XANES.
>> 
>> There are a number of ways you might approach this 
>>problem.  Probably
>> the best way of doing so would be to do a proper fit to 
>>measured data
>> so that you can get a good measure of things like 
>>sigma^2 and e0.
>> Then, using the results of the fit, modify the deltaR 
>>parameters for
>> each path used in the fit to be 3% larger.  Then simply 
>>add up the
>> paths using the fitted values for sigma^2, e0, and so 
>>on.
>> 
>> You can also do something similar without actually doing 
>>a fit to
>> data.  Use Artemis or ifeffit itself to increase R for 
>>each path and
>> simply sum the paths without using fitted values for 
>>sigma^2 and so
>> on.  The easiest way of doing so would be to set deltaR 
>>for each path
>> equal to "0.03*reff".  That will expand the length of 
>>each path by 3%,
>> thanks to the magic of the special "reff" parameter in 
>>Ifeffit.
>> 
>> Another way of doing this (and a decent way of doing so 
>>for a XANES
>> calculation) would be to use Feff's rmultiplier keyword:
>>  http://leonardo.phys.washington.edu/feff/wiki/index.php?title=RMULTIPLIER
>> 
>> 
>> HTH,
>> B
>> 
>> -- 
>> Bruce Ravel  ----------------------------------- 
>>bravel at bnl.gov
>> 
>> National Institute of Standards and Technology
>> Building 535A, Room M7
>> c/o Brookhaven National Laboratory
>> Upton NY, 11973, USA
>> 
>> My homepage:    http://xafs.org/BruceRavel
>> EXAFS software: 
>>http://cars9.uchicago.edu/~ravel/software/exafs/
>> 
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> 
> TaiYen Chen 979-739-7772
> Department of Chemistry
> Texas A&M University
> P.O. Box 30012
> College Station, TX 77842-3012
> 
> Visit us on the web at http://www.chem.tamu.edu
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