Hi,
S is, strictly speaking a valid criterion only under special conditions
which are not fulfilled in "Rietveld". My advice is to forget about the
value of S and think of physics, i.e. bond distances, angles, occupancies
and in favorite cases, also of displacement parameters.
Lubo
On Tue, 3 Jun 2008, Franz Werner wrote:
Dear Rietvelders
I've a basic question on the Rietveld agreement index S (goodness of fit) . In "The Rietveld
Method" (ed. R.A. Young) it says in chapter 1.4 "An S value of 1.3 or less is usually
considered to be quite satisfactory. An S value of 1.7, for example, probably is a warning that you
should look further into the reasons and question the adequacy of your model." In the
literature, however, papers are published quite frequently with considerably larger S values (I'm
just looking at an Acta Cryst. B paper with S values between 6 and 8.5).
The question is now how strict has the 1.3 limit to be applied, what is the "largest
acceptable" S value? How does one assess S>1.7 and justify it to referees?
I should add that I'm not a follower of the R-value cult but think that the
most important criterion is chemical and physical plausibility.
Thanks for your advice.
Franz Werner
--
Der GMX SmartSurfer hilft bis zu 70% Ihrer Onlinekosten zu sparen!
Ideal für Modem und ISDN: http://www.gmx.net/de/go/smartsurfer
