Hi,

  I've only begun to look at pdf, but it seems to me that pdf is only really 
interesting if you want to model non-crystalline material (or 
nano-crystalline), so that there is no long-range periodicity (limited size, 
defects on the borders, large strain, variation in composition,...), and 
therefore the F^2 calculation is not even on option. The reason I've begun to 
look at pdf is that I'm working on a sample with nano-columns that are (at 
best !) 3 or 4 nm in diameter - we're still looking for the Bragg peaks !

  From a computationnal point of view, I think it takes in practice much more 
time to compute the pdf - indeed it is N^2, but N does not even correspond to 
a single unit cell (or sub-unit if centered/centro), but the entire object 
(if nano-sized). If the pdf is computed for a truly crystalline compound, it 
can be reduced to N_asym * N_shell (atoms in the asym unit cell / asym in the 
shell of radius corresponding to the largest d you're interested in).

        Vincent
-- 
Vincent Favre-Nicolin                   http://vincefn.net
Université Joseph Fourier       http://www.ujf-grenoble.fr
CEA/ Institut Nanosciences & Cryogénie  http://inac.cea.fr
ObjCryst & Fox             http://objcryst.sourceforge.net

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