Paul, have you considered explicitly multiple scattering from within the 
tetrahedron?  The short distance you describe for the second shell might 
be consistent with that.

Its pretty easy to write an expression in the fit routine that makes the 
MS path distances explicitly dependent on the SS paths (no new variables 
in the fit)--the number of MS paths is constrained by the geometry 
(tetrahedral).  The order of scattering is probably just 3- and 4- legged 
MS paths, no higher (would give longer distances)

I study arsenic, and so I've seen this approach described in the following 
paper on As: 

Voegelin et al (2007) Geochimica et Cosmochimica Acta v. 71, pp. 
5804-5820.  Look in the section 2.6 for the details.

sorry if someone already answered this,

Andrea L. Foster, Ph.D.
345 Middlfield Rd., MS 901
Menlo Park, CA 94025
T: 650-329-5437
F: 650-329-5491

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