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Re: [sdpd] error measurements
>Are there any views as to which error measurement should be used ? And how
>long will it be before reported powder structures are standardised using
>this measurement, so database authors and more importantly users can
>reliably judge the precision / quality of a structure ?
That question is an endless controversy. However, see §11, page 47 in
McCusker, L. B., Von Dreele, R. B., Cox, D. E., Louër, D. & Scardi, P.
(1999). J. Appl. Cryst. 32, 36-50.
The problem is that if you follow the recommendations, you will
have to give not only some R values, but also a plot of the observed
and calculated data with a scale magnification at large angle for
X-ray data ;-).
About R values, I prefer the conventional Rp and Rwp values
(background subtracted, peak only contributions) plus the Rexp
and the RB and RF values. The X**2 (goodness of fit) can be
calculated from the Rwp/Rexp ratio. But you should not accept
only X**2 without the Rwp and Rexp values, IMHO. A problem
can be that people give Rp and Rwp without background
subtraction, and without even saying it.
And what about structures refined with many restraints and/or
constraints and with the (number of reflections)/(number of
refined parameters) quite small (< 3 or even < 1) ? They
can give very good R values while being somewhat dubious.
At least, they are more dubious than structures refined with
such a ratio > 10. Thus, giving that ratio is an important
information too.
Hope this help.
Armel Le Bail
http://www.cristal.org/course/