>I have another question now. In GSAS, there are two methods to extract the
>"|Fobs|"s. One is Le Bail method, and the default is "normal" method. The
>"normal" method apportions the observed Bragg intensity above background
>according to the portion of the total calculated intensity contributed by each
>reflection to give a set of "observed" structure factors with phases. My
>question is what the differences is between the "normal" method and Le Bail
>method?
Just to set the record straight, without commenting on GSAS in particular.
Already in the original Rietveld program, the 'observed' Bragg intensities (or
structure factors) were estimated by dividing the intensity at each profile
point according to the calculated structure factors. This is more useful than
it might seem :-) since by iteration of difference Fouriers, such ESTIMATED
'observed' Bragg intensities can be used to obtain a more complete model for
Rietveld refinement eg to locate hydrogen atoms in neutron diffraction. (The
original purpose was just to estimate the conventional Bragg R-factor).
A second method was invented by Pawley while Armel was still a young man :-)
Pawley, G. S. (1980) J. Appl. Crystallogr. 13, 630-3.
"EDINP, the Edinburgh powder profile refinement program ."
In this Pawley method, the individual Bragg intensities are refined to fit the
full diffraction pattern, a kind of "profile refinement" that does not require a
model for the structure, and that produces a less biased estimate than does
the original Rietveld method ("normal method").
The Pawley method was then improved by le Bail, and it is now Armel's code
that is commonly used.
Alan H.
Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13
ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
