RE: Standards, Rietveld Bragg R and background

VONDREELE@mist.lansce.lanl.gov
Tue, 12 Aug 1997 9:02:39 -0600 (MDT)

Armel (and others),
>The problem is that some softwares give only
these meaningless total Rp and Rwp.
I beg to differ. The function minimized by the least squares in a Rietveld
refinement is M=sum[w(Io-Ic)^2] and Rwp is defined as sqrt{M/sum[w(Io)]}. Nothing
is more clear than that. The other residuals are interesting and sometimes useful
for characterizing how this residual is distributed between Bragg intensities and
background but they are not directly related to the minimization function. The
subtracted background R is "flawed" because it assumes that all the random error in
the measurements belongs to the Bragg contribution and that the (subtracted)
background contribution has none. Thus, it is a over estimate of the contribution
to the residual from the Bragg intensities. I think that the real point in all this
is that R-value comparisions between different classes of diffraction experiments
(x-ray vs neutron TOF, etc.) is particularly meaningless because the differing
background contributions, scattering factors, Lorentz factors between the
experiments have such an enormous impact on the quality of the refinement results.
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.
Bob Von Dreele