Hi Paul,

I think we're getting confused over what we each mean by "interfere." Whether sites are equivalent or inequivalent, neighboring or far apart, the wave functions do not interfere at all, in part because the odds of two x-ray photons being absorbed by sites close to each other at nearly the same time is very very small--if it wasn't, your material would disintegrate in an instant!

The idea behind the path expansion used in EXAFS analysis by ifeffit, however, is that the chi(k) for each possible scattering event by a single electron can be computed separately, and then the results added (I'll defer to John or Matt or whoever for a clear explanation of why this is justified). In other words, the EXAFS spectra can be thought of as if there were a contribution from an electron scattering off a near-neighbor, another scattering off a 2nd-nearest-neighbor, another doing a multiple-scattering thing, etc. (all weighted, of course, by the relative contributions of these scattering events). These are literally just added. Given that system, if there really are multiple sites that are each having their own scattering events, then it is perfectly appropriate to treat them in exactly the same way--just add. So, depending on your terminology, either we are combining all scattering paths coherently or we are adding none of them coherently, regardless of whether they correspond to the same absorbing site or not. It would not be correct to add, say, the magnitudes of the FT's for the separate contributions...the phase of the FT indicates where the peaks and troughs are in k-space (and thus energy-space), and it certainly matters whether a given scattering path has enhanced or suppressed absorption at a given energy!

Hope that helps...

--Scott Calvin

Sarah Lawrence College

P.S. I've published several analyses of systems with inequivalent sites. If you'd like a pdf of one of those papers, let me know.