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
