[Ifeffit] SS and MS contributions to EXAFS

Scott Calvin dr.scott.calvin at gmail.com
Sat Jun 18 10:40:49 CDT 2011

Hi Francisco,

Following up on Matt's comment, a FEFF calculation  with default  
option assigns sigma2 to be 0, and that probably won't result in  
relative contributions between SS and MS that look much like what  
happens in the real material.

A better strategy would be to read the calculation into Artemis, as  
Matt suggests, and use a Debye model for the sigma2 for the paths,  
trying a few different Debye temperatures to get a sense of how it  
affects the relative contributions. While the Debye model is not an  
appropriate model for quantitatively fitting most materials, at least  
it takes a stab at how sigma2 might depend on R, which will help with  
understanding how important the MS paths are to chi(k).

--Scott Calvin
Sarah Lawrence College

On Jun 17, 2011, at 7:21 PM, Matt Newville wrote:

> Hi Francisco,
> On Fri, Jun 17, 2011 at 7:34 PM, Francisco Garcia
> <garcia.ff.000 at gmail.com> wrote:
>> Dear users,
>> I would like to quantify the single scattering (SS) and multiple
>> scattering (MS) contributions to the EXAFS spectra over a range of k
>> values. I adopted the following approach and I would like ask
>> experienced users if my approach is sound:
>> (1) Run a regular EXAFS (default NLEG=8); I assume this includes all
>> possible scattering paths. Call the chi(k) data chi.dat
>> (2) Run another EXAFS but this time I set NLEG=2. I assume this is  
>> for
>> all SS contributions. Call the chi(k) data chi_ss.dat
>> (3) To obtain the MS contribution, I subtracted chi_ss.dat from  
>> chi.dat
>> If my approach is faulty, can you tell me how to remedy it?
>> Thank you.
> That approach would work under the assumption that the atomic
> coordinates in the feff.inp file fully described the distribution of
> atoms in the system.  If, on the other hand, the feff.inp had an
> idealized structure, then the approach would model the MS
> contributions for that structure.  For example, a feff.inp file often
> assumes no static or thermal disorder in the system.  In that case,
> the MS contributions would have no disorder terms (eg, sigma^2)
> applied to them.
> An equivalent approach that may be somewhat simpler would be to use
> Artemis to run the Feff calculation with NLEG=8 (4 is probably good
> enough below 6Ang in most structures),  read in all the paths, and sum
> the paths with NLEG>2.
> --Matt
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