>In case any one was wondering...
>>Then, if I am not wrong, I have one question : who invented the
>>total Rp and Rwp factors ?-) I suspect that they were invented
>>by error or just for an endless discussion about their absence of
>>meaning regarding the crystal structure...
>These things were originally described by Hugo Rietveld. Rpw in particular
is
>statistically "unassailable" as it is directly linked to the function that
is
>minimized in a Rietveld Refinement.
Yes Bob, sure that Rp and Rwp were originally described by Hugo
Rietveld.
But.
Remember that the background was not originally described by
functions with refinable parameters. So that the function practically
minimized (i.e. in the original softwares) was not including the
background. And this was logical.
Have a look at fig. 4 in J. Appl. Cryst. 2 (1969) 65-71 by H.M.
Rietveld and observe the zero background, a fact which is
certainly not experimental for CaUO4 on any neutron diffractometer.
So that, even if this is not explicitly written in the paper, the yi(obs)
in the Rp formula is in fact Iobs-background. Remember the first
versions were in two steps, one for preparing data and one for
the refinement. The background was subtracted in the first step
(or am I wrong ??). This background corresponded to values
interpolated between intensities estimated manually at a few
angular values.
So that the "total Rp and Rwp" (i.e. including background and points
where no reflections are contributing) were probably introduced by
those having included the background as described by refinable
parameters. May be it was for ToF data? The question remains
interesting.
If you remove the sample, the background is still there... This is the
best way to explain why the total Rp and Rwp are not a lot related
to the compound structure. The more the background is high and the
more the total Rp and Rwp are low if the background is well fitted :-).
Hence the old (continuing) controversy. I systematically give the
conventional Rp and Rwp in all my manuscripts, indicating that they
are "background subtracted" and "peak only". I never mention the
total Rp and Rwp because their meaning is null for the structure-
related intensities. The problem is that some softwares give only
these meaningless total Rp and Rwp so that their users have no way
to access to the conventional R values (if you want a list of these
softwares, let me know).
So who invented first the "total Rp and Rwp" ?? This don't seems to
be H.M. Rietveld, IMHO. Or maybe the old software I just had a
look inside was not the original ?
Armel