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RE: [sdpd] Help Request
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<p>In simple terms, the correlation matrix is a matrix of numbers that<br>
relates the dependence of refining one parameter on another. Usually it<br>
is written in percentages where 100 means that the two parameters in<br>
question are 100% correlated i.e. changes in one parameter follow<br>
changes in the other. For instance if one takes the parameters UVW often<br>
used to define the angular behaviour of peak widths, it is almost always<br>
the case that if one looks at the appropriate columns and rows of this<br>
matrix one finds that they are something like 98% correlated. This just<br>
means that one cannot be sure of the absolute values of any of the<br>
individual refined parameters as they depend so highly on each other. If<br>
one gets a highly negative correlation term then that means that the<br>
least squares shifts of one parameter tend to be opposite to the other<br>
(one goes up and the other down). The difficult question then is what<br>
constitutes a high correlation value, sufficient to throw doubt on the<br>
refined result? Obviously 80-100 is high, but what about 60%? It would<br>
be interesting to know what most people consider to be sufficient to<br>
cause concern.<br>
<br>
None of this has anything directly to do with the R-factors which<br>
attempt to give you some idea as to how well the model you are using<br>
fits the actual powder diffraction data. <br>
<br>
How to judge whether the refinement is good? Now that is a real thorny<br>
problem that has never been satisfactorily answered. There are cases of<br>
really low agreement factors that do not necessarily mean that the<br>
refined structure is correct. In the end the only thing one can sensibly<br>
do is to look at the refined structure and ask yourself whether it makes<br>
chemical and physical sense (e.g. acceptable bond lengths, bond<br>
valences, agreement with known physical properties - although even here<br>
one needs to be aware that your refinement may be telling you something<br>
different from the accepted chemistry which in turn may indicate<br>
something important). How for instance can one distinguish between the<br>
correctness or not of refining a structure in one space group and then<br>
refining it again in a lower symmetry space group, especially if one is<br>
dealing with pseudosymmetric structures such as perovskites? Both sets<br>
of agreement factors may well be similar. Let us remind ourselves again<br>
that with Rietveld we are refining crystal structure parameters using a<br>
1-dimensional set of overlapping data rather than using individual<br>
structure factors from a single-crystal 3-dimensional set of data, and<br>
so interpretation of the refinement must be done with caution (and<br>
sufficient open-mind and a readiness to change the model in the light of<br>
any subsequently new information that may be discovered).<br>
<br>
Mike Glazer<br>
<br>
From: <a href="mailto:sdpd%40yahoogroups.com";>sdpd...@yahoogroups.<wbr>com</a> [mailto:<a href="mailto:sdpd%40yahoogroups.com";>sdpd...@yahoogroups.<wbr>com</a>] On Behalf Of<br>
bensemma nouar<br>
Sent: 04 March 2010 08:30<br>
To: <a href="mailto:sdpd%40yahoogroups.com";>sdpd...@yahoogroups.<wbr>com</a><br>
Subject: [sdpd] Help Request<br>
<br>
Hi<br>
I would like to know what the correlation matrix tell us? and how i can<br>
juge that my refinement is good because i found agreable Rb and Rf in<br>
contrast relatively high value of goodness of fit and the others<br>
profile jugement factors ?.<br>
<br>
my best regards<br>
<br>
[Non-text portions of this message have been removed]<br>
<br>
[Non-text portions of this message have been removed]<br>
<br>
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