Hi Peter, On Wed, 14 Jan 2004, Peter Southon wrote:
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.
Shelly gave excellent advice for how to deal with this problem, but the overall answer to 'is there a general method for defining MS Debye-Waller Factors' is still no.
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.
There has been some work to use a set of force constants to generate MS DWFS, and attempts made to generate these from (close to) first principles. There was work along these lines by A. Poiarkova and Rehr (including Poiarkova's thesis) which describes generating DWFs this way. Probably A Poiarkova and JJ Rehr, PRB 59 948-957 (1999) is the best reference. This approach aimed to calculate the DWFs, not to necessarily model/fit them. This work could, in principle, be turned around to allow a parameterized fit using a few force constants that generated the appropriate sigma2. Similar work has been done (or is being done) by N. Dimakis and G. Bunker (I'll probably get in trouble for calling them similar, as the approaches are pretty different, but both aim to calculate EXAFS DWFs from nearly first principles). I believe N Dimakis and G Bunker, PRB 65 201103(R) (2002) is the place to start. To me, this work seems to be a little more straightforward to parameterize for a fit, but it would be real work to try to put such capabilities into ifeffit. I don't have a good feel for how general either of these approaches is or what the prospects of using them "in general" is. Hopefully John or Grant can correct anything I've misrepresented!! It sure would be nice to have something like this available in ifeffit, wouldn't it? --Matt