[Ifeffit] Question about FEFF6

goodhei8 at gmail.com goodhei8 at gmail.com
Tue Apr 12 02:24:22 CDT 2011

Dear Prof. Ravel,

Thanks so much for your answer, which remove the cloud partially above me.

I actually tried many models, from crystal to the opposite 6O only 
(different distance, second shell C,P ...).  The example shown in the 
email are two extreme cases, which single P has contribution like 6 P.  
I also tried 6O+6(PO3), which is close to a crystal structure(6 O are 
not same distance from center, same for 6(PO3)) ---- when I decease # of 
PO3 from 6 to 0, FEFF calculations of O change gradually --- not like ' 
single P has contribution like 6 P'. It make me wonder,

1, FEFF takes Z atoms at equal distance as same. It does not care some 
have neighbor A, some have neighbor B. Should I worry about it? and when?
2, Does FEFF consider about valence state (like Ur 3 or Ur5 in center)?

Model closer to sample, better result. Of course crystal structure is 
closer to real sample than models like 6O do. In practice, If I only 
consider single scattering and fit to n shell, can I say that it is good 
enough to have a model of n+1 shell? For instance I want to fit sample  
with O and P shells, it is good enough to have a model with Cd -O-P-O3, 
since O3 shell will influence P(like P atoms influence 1st shell O).

Jack Song

On 2011-04-11 15:05, Bruce Ravel wrote:
> Jack,
> You have rediscovered the conventional wisdom about properly bounding
> a cluster for calculation with Feff.  When there are not enough atoms
> on the periphery of the cluster to constrain the size of the muffin
> tins, the muffin tins tend to be much too large.  As you can see this
> has consequence for the scattering function calculated by Feff.
> In your case, the muffin tins of the oxygen atoms are about 0.73
> Angstroms when including even a single P atom on the periphery of the
> cluster.  For the first calculation Rmt is 1.06 -- substantially
> larger.
> What's interesting is that a single P atom is sufficient to make the
> calculation of the O contribution work out reasonably well.
> Have you read Shelly's book chapter review of EXAFS?  In her example
> on determining a second shell atom type, she runs Feff calculations on
> known crystal structures with various second shell atoms.  She then
> takes just the kinds of paths she needs to model different
> contributions to the EXAFS.  The reason she adopted this strategy
> rather than trying a small, notional cluster of the sort the you have
> made in your example is because constraining the muffin tins is quite
> important.  Indeed, the potentially counter-intuitive strategy of
> snarfing paths from some related crystal structure works well
> precisely because it is a way of guaranteeing sensible muffin tin
> radii.
> HTH,
> B
> P.S. You can see the muffin tin radii by opening the .apj file as a
> zip file (which is what it is!) and looking at the misc.dat file in
> the dataX.feffY folders.
> On Monday, April 11, 2011 02:33:54 pm goodhei8 at gmail.com wrote:
>> Dear IFEFFIT Members,
>> I have a strange problem with FEFF calculation. I am not sure if I made
>> some mistakes in FEFF of input file, or FEFF did not handle it well.
>> For Oxygen paths calculated with FEFF in Artemis by 3 input files, the
>> results for O are supposed to be close, but I got a big difference, when
>> I transform them into r space(k 2.5~10,kweight=2 in attachment).
>> There are three input files for FEFF:
>> #1, 6O only on xyz axis
>> #2, + 1P besides 6O
>> #3, + 6P besides 6O
>> For O, #2 and #3 are close(almost same), but #1 is much bigger. (It
>> would make sense to me if #1 #2 are close.)
>> Thank a lot.
>> Best,
>> Jack Song

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