[Ifeffit] Debye-Waller factors for MS paths

[Matt Newville]

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