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

[Scott Calvin]

Ying and Bruce,

Some versions of FEFF, at least, do allow amps above 100. They assign  
100 to the first path, and then scale from there. The nearest neighbor  
path in your list is a single oxygen. I didn't run atoms on your input  
file, but I bet your path 14 is some combination of high degeneracy,  
focused, and involving the lead. Despite being further, it is more  
prominent, and thus correctly assigned a higher amp.

It may be that this is true for only some versions of FEFF. I've  
certainly seen it frequently, although I haven't paid attention to  
whether it's version or even platform dependent.

Personally, despite being documented, I would describe the behavior  
that Bruce describes, where the meaning of amp gets rescaled if a more  
prominent path is encountered, as a "known bug," while the behavior  
where amps greater than 100 assigned as more ideal. In any case, this  
is just cosmetic; it shouldn't affect the feff path itself.

--Scott Calvin
Sarah Lawrence College

On Aug 29, 2008, at 9:32 AM, Bruce Ravel wrote:

> 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