[Ifeffit] Problems with feff and crystallographic data (Now with bug report)

[Carlo Segre]

Hi Hashem:

As Gregory mentions below.  The problem is partial occupancies.  In 
diffraction, it is OK to put two different atom species in the same 
(or close) crystallographic position.  This makes no sense in analyzing 
EXAFS because you cannot calculate scattering from a fractional atom.

For EXAFS, you need to decide what the specific local environment of your 
target atom will be.  If you have two different kinds of environments, 
then you have to do two (or many more) separate FEFF calculations and then 
fit to both of them simultaneously to get the effect of partial occupancy.

Cheers,

Carlo

On Fri, 19 Dec 2008, Gregory Merchan wrote:

> Hi Hashem,
>
> Atoms doesn't handle the occupancy data in the CIF file. See this page for
> more info:
>
> http://cars9.uchicago.edu/iffwiki/FAQ/FeffitModeling#head-b273ecfd19542ec82ea031bbcc57ff8861fc11f9
>
> I assume you see a message like this one that I see:
>
>    This warning indicates a problem in the Atoms list.  For some
>    reason you have atoms that are separated by than 0.93 Angstroms
>    (or 1.75 Rydberg).
>
> If you look on the Atoms tab in Artemis when you open the CIF file, you
> should see that the co-ordinates
> for Mg and Al are all 0.00000 ; i.e., all Atoms sees is that they are in the
> same place.
>
> On Thu, Dec 18, 2008 at 10:04 PM, Hashem Stietiya <mstiet2 at lsu.edu> wrote:
>
>>  Hello!
>>
>>
>>
>> I was using Artemis to run feff and generate paths from hydrotalcite.
>> However, it failed to run and an error message indicated that there is
>> something wrong with the crystallographic data. I used the crystallographic
>> data of hydrotalcite from different sources, but encountered the same
>> problem. Can you please help identify the problem and run feff?
>>
>>
>>
>> Please find attached the crystallographic data for hydrotalcite.
>>
>>
>>
>> I appreciate your help.
>>
>>
>>
>> Regards,
>>
>> Hashem Stietiya
>>
>> Louisiana State University
>>
>>
>>
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>>
>>
>

-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Special Projects, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu   http://www.iit.edu/~segre   segre at debian.org