[Ifeffit] a cautionary tale

[Peter Southon]

Hello fellow Ifeffiters

This a story about making feff.inp input files for FEFF. The more 
experienced readers of this list will probably be well aware of the issues 
involved, but I thought that relative newcomers such as myself might not 
know (which is why I'm posting this on the Ifefft list rather than the Feff 
list).
In a nutshell, it is wise to create a large cluster of atoms to put into 
Feff (ie use a large Rmax in Atoms), but limit the number of paths with a 
reduced Rmax in Feff.

I used Artemis/Atoms to make up a Feff input file (feff.inp) from 
crystallographic parameters, setting Rmax in Atoms to a relatively large 
value of 7.5 Å to create quite a large cluster of atoms. I then used FEFF 
to calculate scattering paths within the cluster, but first reduced the 
Rmax in feff.inp to 5.5 Å to limit the number of paths. So far so good.

I then (for reasons too involved to discuss here) edited the feff.inp file 
to delete all atoms more than 5.5 Å from the centre, and ran Feff again. 
The results (judging from the relative amplitude values) were different, 
and in the case of some paths very different. Some intense paths changed in 
amplitude by a factor of 2 or greater, although I haven't looked at the 
finer details of the Feff results. The paths that changed the most were 
high-order multiple scattering paths, and surprisingly those paths did not 
involve atoms at the edges of the 5.5 Å radius.

Now I'm sure that someone else can explain the reasons for this much better 
than I can, but I assume that the potentials for the atoms outside the 5.5 
Å radius (which were neglected in the second calculations) significantly 
affect those inside the radius.
I know that its always been my practice to perform the Feff calculation 
using only those atoms that I think are contributing to my spectrum, and 
I'm sure that others do the same. My advice is to use a larger cluster of 
atoms in Feff if you can - but as to how large I might leave that to 
someone else to comment.

Regards,
Peter

PS - and four cheers to Bruce for improvements to Artemis, I can't wait for 
the Windows update.



Peter Southon
Post Doctoral Fellow
School of Chemistry
University of Sydney
NSW 2006, Australia
+61 2 9351 4425