[Ifeffit] Re: Artemis Question

[Scott Calvin]

Shelly said:

>
>Feff is truly amazing.

Amen!

>1) Feff needs to have a list of atoms that extends well beyond the 
>paths that are actually used in the fit.  Always pad the feff.inp 
>file by at least an Angstrom.  If you are using feff calculation for 
>the first time.  Pad the feff list by one angstrom and then by two 
>angstroms and check that the paths that you are going to use are the 
>same.  You do not want to use paths from the edges of the cluster 
>that you give to feff in the feff.inp file.  -Hence you need to use 
>a structure like a metal-oxide to model a hydrated metal.

Under what circumstances does FEFF actually need these extra atoms? 
When I take the copper example included by Artemis, for example, I 
can rerun the FEFF calculation with a cluster size of 3 angstroms, so 
that FEFF only has the nearest-neighbors to deal with. The path 
generated is identical to when a cluster size of 7 angstroms. 
Certainly if someone is using FEFF8 with a self-consistent field 
calculation it is necessary to have the cluster extend beyond the 
furthers paths, but with FEFF6L is this true?

(Incidentally, for the novices out there, I do NOT recommend being 
satisfied with a single-shell fit when you have a reasonable guess as 
to the structure beyond the first-shell. One reason can be seen 
easily with Artemis: paths beyond the first-shell often have low-R 
"tails" that contribute to the signal even at the first-shell peak. 
Although the FEFF calculation for the first path may be the same 
whether or not you also calculated paths further out, the Ifeffit fit 
will be different depending on whether you include those "outer" 
paths or not.)

--Scott Calvin
Sarah Lawrence College