[Ifeffit] Debye-Waller factors for MS paths

[Peter Southon]

Dear everyone,

One of the features of constructing models with Ifeffit is that we are 
forced to think long and hard about the parameters to use for each path. In 
particular I have had a great deal of difficulty in deciding how to 
approach the Debye-Waller factors for with multiple scattering (MS) paths.

The ideal approach is to define the D-W factors for the MS paths in terms 
of the factors for the single scattering paths. That way you add no further 
variables to the model. But if you think about it you will realise that you 
cannot simply add all the relevant SS factors up to get a sum for your MS 
path - it all depends on the geometry, the direction of vibration, 
correlation of vibrations, etc. Its all a bit tricky for those of us 
dealing with intense MS paths.

So what I am asking is, does anyone know of a general method or approach to 
define the Debye-Waller factors for MS paths (in terms of SS paths or 
otherwise)? How do all you Ifeffiters deal with the problem? I've found a 
few papers that discuss it (listed below) but they generally assume that 
all vibrations are uncorrelated, which is seldom a realistic situation in 
continuous solids.

Munoz-Paez, A. (2000). Inorganic Chemistry 39(17): 3784-3790.
Sakane, H. (1998). Journal of the American Chemical Society 120(40): 
10397-10401.
Haskel, D. (1998). PhD thesis, University of Washington.  (appendix)
         http://www.aps.anl.gov/xfd/people/haskel/PS/thesis.ps

Is there anything out there that I've missed? I'd be particularly 
interested in anything relating to continuous solids or correlated 
vibrations. Comments/questions/pearls of wisdom very welcome.

Peter


Peter Southon
Research Fellow - School of Chemistry
University of Sydney, NSW 2006, Australia
+61 2 9351 4425