Debye-Waller factors in FEFF

[Scott Calvin <scalvin@anvil.nrl.navy.mil>]

Hi all,

I am working with a material that appears to be a moderately to 
highly disordered FCC compound. Naturally, a large Debye-Waller 
factor has to be applied to the paths generated by FEFF to achieve a 
good fit. The difficulty is that it seems to me that under these 
circumstances FEFF dramatically overestimates the relative amplitude 
of focused paths; focusing should be much more disrupted by disorder 
than either direct scattering paths or multiple scattering paths with 
other geometries, such as triangles. Assuming you agree with the 
previous statement (if not, please let me know!), I can think of a 
number of ad hoc strategies for dealing with this problem: moving 
atoms slightly off their lattice sites by an arbitrary amount prior 
to the FEFF calculation, suppressing focused paths altogether in 
performing a fit, assigning the focused paths a Debye-Waller factor 
greater by some arbitrary (or fitted) factor than non-focused paths, 
etc..  All of these methods are either arbitrary or undesirable for 
other reasons; does anyone have a more elegant method of doing this 
with FEFF (and/or fitting programs such as FEFFIT)?

--Scott Calvin
Naval Research Lab
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