[sdpd] Re: Rietveld Method Definition

[Armel Le Bail <alb... @cristal.org>]

Dr. Arnt Kern wrote:

>With this contribution we are going to present some evidence, that the
>strict distinction between structure refinement (particularily Rietveld
>refinement) and structure solution is loosing its base. Moreover we will
>proove, that the following commonly stated "characteristics" of the Rietveld
>method are no longer true in this rigid form:
>
>- "...the Rietveld, being nothing else than a complex minimization
>procedure, cannot be taken as an active tool, since it can only slightly
>modify a preconceived model built upon external knowledge."
>- "The starting parameters must be reasonably close enough to the final
>values."
>- "The sequence into which the different parameters are beeing refined needs
>to be carefully studied..." 
>(Masciocchi, N., 1997, Rigaku Journal)

In a next step, you may also try to include indexation in your
whole stuff, and call that "Rietveld method". And why not synthesis
of the compound ? Masciocchi's definition is excellent and will
stay untill studying powder will lose sense because taking the
smallest grain of that powder will be sufficient for a structure
determination and refinement by electron diffraction.

>What about the following model:
>
>site Pb  x  0.0  y  0.0  z 0.0
>....
>site O   x  0.0  y  0.0  z  0.0
>
>Actually this is still a model. And if a "Rietveld program" sucessfully
>refines this model to the true coordinates this is * structure solution *.
>Isnt it? 

Yes, but this is not Rietveld (Hugo, help me, or ask
for being co-author of their paper !-).

>By the way: WITHOUT preceeding intensity extraction in a separate
>step! 

OK, you added a subroutine or two...

You explain the WITHOUT, but you do not say any word 
about the WITH. And the WITH is not the Rietveld method.
We can guess what it is, even if you don't tell us, you now ;-).
Your surprise effect is a flop. Why waiting for Glasgow ?
What is done in the subroutines ?
Put a cross at the good answer, please :
       Simulated Annealing ?
       Monte Carlo ?
       Genetic Algorithm ?
       Molecular Replacement ?
       Molecular Modelling ?
       Use of Interatomic Potential ?
        ... ?
       mixing of all the above ?
       Completely New Science ?
       Structure Prediction ?
       Packing Considerations ?

Thanks.


Armel Le Bail - Universite du Maine, Laboratoire des Fluorures,
CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France
http://www.cristal.org/