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
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