Date: Mon, 25 Aug 1997 10:39:11 +0200
To: RIETVELD_L Distribution List [rietveld_l@ill.fr]
From: Armel Le Bail [armel@fluo.univ-lemans.fr]
Subject: RE: Standards, Rietveld Bragg R and background
Bob wrote:
>What really matters is the quality of the structural result (atom positions,
>temperature factors, bond lengths, angles, etc.) and how they compare with
other
>known data and not whether Rwp is such-and-such a value.
Hum, Bob, I have just received the CPD Newsletter No18 in which I
read "The commission is following up the Rietveld refinement comparison
study published by Hill et al in 1994 with a set of recommendations. This is
being led by Lynne McCusker and Bob von Dreele and will probably appear
in one of the IUCr journals this year". So, if I understand well your
point, the R
values do not really matter and this could be your recommendation to let users
propose any definition of R in their manuscript ?
Most Journals give informations to authors. For instance, the J. Solid State
Chem. ask for the "definitions and final values of R factors (weighted and
unweighted) [e.g. RI (RF), Rwp, Rexp, S]". I would add Rp to the list. Only
few of these were provided by the last GSAS version I used. Even RI was
not available. I am sorry but my opinion is that a Rietveld software should
at least provide the Rs as defined by Rietveld (background subtracted). Now
there is no opprobrium to those who want to add other R definitiions
(background included).
I agree with Alan writing :
>Hugo Rietveld didn't include the background in the definition of the
>profile R-factors, and that choice IMHO appears to be the best. He did
>correctly include the background in calulating the statistical error for
>each point (the weighting scheme), and he did correctly calculate the
>estimated errors in the parameters. That is much more important than
>simply obtaining "low R-factors".
Anyway, I would like to obtain from the CPD some clear recommendations
about which Rp and Rwp should be given. Hearing that the conventional
Rietveld Rp and Rwp values should be given in any case would make me
happy. Hearing that if some want to give additionally the DBW-Young Rp
and Rwp values (background included), this is wellcome too because the ratio
(Rietveld Rp) / (DBW-Young Rp)
will give an idea about how high was the background.
In many publications, Rp and Rwp are given without cleardefinition. You may
be sure that peoples will choose to give the DBW-Young R values because
they are systematically lower than the true Rietveld ones. The worst I
have seen is in papers giving only the GoF Chi2 values, which were all near
of 1 and no figure were given showing the observed and calculated patterns.
In fact, Chi2 = 1 could mean that Rwp=35%=Rexp. In such a case, making
another pattern with longer exposure would be the thing to do before
submitting the publication...
Obtaining "low R-factors" may not be very important however,
obtaining "low R-factors" together with "low FWHMs" is significant.
Has somebody an idea for a new R definition including both aspects ?
Imagine a conventional Rietveld Rp = 5% together with FWHM
as low as 0.01 2-theta degrees. How to distinguish this from a result
from another experiment leading to the same Rp but with FWHM=0.15 ??
Last point on the Rietveld Refinement Round Robin. The questionnaire
for structure refinement of PbSO4 asked for the Rp, Rwp, Rexp, GoF
and RB (=RI) definitions (how many participants had indicated if the
background was subtracted or not??) but the form for reporting the
M-ZrO2 refinement results only asked for the R values. Few people
have thought to indicate if the Rp and Rwp values they gave were either
the true Rietveld ones or the DBW-Young ones. Fortunately the
reanalysis provided both Rietveld and DBW-Young Rp and Rwp values.
The recommendation given in the RRRR conclusion was :
"it is desirable that both types of agreement indices be calculated
and subsequently quoted in published work in order to permit
valid comparisons to be made between instruments with different
PtB ratios and using different wavelengths".
My hope is that the CPD will not recommend now the contrary.
Following the above recommendation, the first you have to do, Bob,
I am afraid, is to include the true Rp and Rwp in GSAS together with
RB :-).
Armel
P.S 1. In 1992, I asked for the compilation on the RRRR PbSO4
results deposited with the British Library Document Supply Center
(supplementary publication No SUP 55098). The document had
108pp, most of them completely white. Probably an electronic
error ??
P.S. 2. May I suggest that Rp and Rwp values given in published Rietveld
refinements should be accompanied by the labels :
"DBW-Young" if including the background
and "Rietveld" (or nothing at all) if excluding the background
with no supplementary indication should be the conventional
ones (background subtracted) and that the "background included" values
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