# [Ifeffit] Comparison of "amp" between Feff8.40 and feff6 - Curved waves or Plane waves?

Bruce Ravel bravel at bnl.gov
Fri Aug 29 08:32:32 CDT 2008

```On Thursday 28 August 2008 18:42:12 Ying Zou wrote:
> While I am calculating FEFF path through Feff8.40 using the "feff8.inp" as
> attached, I notice that the value of "amp" shown on "FEFF interpretation
> page" for some paths goes over 100%. For example, path 14 has a Reff=3.9317
> far away from first shell, yet it has an "amp"=169.913. On the contrary, if
> I do the same in Artemis through embedded Feff6.0(using "feff6.inp"),  none
> of paths has an "amp" over 100%. I am guessing this could be all right
> because Feff8 is using curved waves Feff6 instead plane waves. Any comments
> on this would be greatly appreciated!

Ying,

Two things:

1. Both Feff6 and Feff8 are a curved wave calculations

2. From the Feff6 document (the definition is unchanged in Feff8, to
the best of my knowledge, although this definition is not included
in the Feff8 document):

The curved wave importance ratios are the importance of a
particular path relative to the most important path encountered
so far in the calculation.  If the first path is the most
important in the problem, all the importance factors will be
expressed as a fraction of that path.  However, if the third
path considered is the most important, and the first path is the
next most important, path 1 will have a factor of 100%, path 2
will be a fraction of the first path's importance, path 3 will
have an importance of 100% (since it is now the most important
path), and subsequent paths will be expressed as a percent of
path 3's importance.

So, an amp of 170 for path 14 must be a bug in Feff8.  By its own
definition, 100 is the largest possible value.

I suspect that if you compare that path from the two Feff calculations,
they will, in fact, be quite similar.

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
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