[Ifeffit] What does FEFF stand for?

[Scott Calvin]

I looked into this a bit further, Bruce, and I'd tentatively say the  
curved-wave corrections do turn out to be the source of the "eff":

The earliest use of f_eff I can find is from a 1986 Phys. Rev. B  
article entitled "Spherical-wave effects in photoelectron  
diffraction," by Sagurton et al. (John Rehr is also in the author  
list). It says "an approximaton for including SW [spherical wave]  
corrections suggested recently by Rehr, Albers, and Natoli has been  
incorporated in some of our calculations...the net limiting result is  
a calculation procedure in which an effective scattering factor  
f_eff,j(r,theta_j) which depends on r takes the place of the usual PW  
[plane wave] f_j(theta_j)."

In addition, was FEFF3 a multiple-scattering code? The comments in its  
header and the 1991 JACS article on it mention only single-scattering.

It would make an extraordinary amount of sense that the "eff" would  
refer to FEFF's ability to handle multiple-scattering paths, but I  
don't think that's the actual historical origin of the terminology.

And as for Anatoly's suggestion, I'll, uh, leave that one be for the  
moment.

--Scott Calvin
Sarah Lawrence College

On May 10, 2011, at 12:17 PM, Bruce Ravel wrote:

> On Tuesday, May 10, 2011 03:03:23 pm Scott Calvin wrote:
>> My understanding, although I could be wrong is that the "effective"
>> part came from an improvement of the theory to account for curved- 
>> wave
>> effects. In other words, early theories approximated the  
>> photoelectron
>> as a plane wave, but of course it spreads out radially from the
>> absorbing atom. That change necessitated tweaking the definitions of
>> the factors, so it became the "effective" f.
>
> I think you are mistaken.  My memory of the etymology has to do with
> the formalism dating back to Feff5 for computing MS paths.
>
> For a purely single scattering theory, you have an F and a phi
> (without the subscript eff).  That is, you can simply compute the
> scatting function for the one scatterer and be done with it.
>
> Feff's path expansion introduced two clever things to the EXAFS
> business.  One is that it provided a formalism for computing a single
> function that takes into account the angle-dependent scattering
> functions of all atoms in an arbitrary-geometry multiple scattering
> path.  This allows one to treat a MS path with the familiar SS EXAFS
> equation only by replacing F and phi with F_eff and phi_eff.  That
> innovation is central to how Ifeffit works.
>
> The second clever thing is that it's really fast.  That's not such a
> big deal today, but back in the mid-90s, when a Feff run could take
> several minutes, a faster algorithm was very welcome indeed.
>
> B
>
>
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