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