[Ifeffit] Question about FEFF6

[Bruce Ravel]

On Tuesday, April 12, 2011 03:24:22 am goodhei8 at gmail.com wrote:
> 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?

In Feff6, two atoms that are at the same distance and have the same
unique potential make the same contribution to the EXAFS.  I don't
quite understand what you want to worry about.

> 2, Does FEFF consider about valence state (like Ur 3 or Ur5 in center)?

Feff6 does not understand valence.  This is one of the differences
between Feff6 and Feff8.  Feff8 uses a self-consistency loop to
transfer charge between the atom types, resulting in something that
can be related to formal valence in some situations.

The follow-up question you have to ask is whether EXAFS analysis is
actually sensitive to charge transfer.  In the regime in which the
EXAFS fit is evaluated, the photoelectron has large kinetic energy and
is not particularly sensitive to the details of the potential surface.
In almost all situations, Feff8 is a modest improvement at best over
Feff6 for EXAFS analysis.  That is why we continue to distribute and
use Feff6 with Ifeffit and Artemis.

Of course, charge transfer is a dramatic effect in the XANES.  But you
were asking a question about EXAFS analysis.

B

-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/