Dear Olga,
As you've noticed, the Rietveld R-factors tell you how well you fit the
pattern, but don't tell you how well you fit the peaks. There is a
correlated intensities R-factor which follows from the methods outlined in:
J. Appl. Cryst. (2004). 37, 621-628 [ doi:10.1107/S0021889804013184 ]
"On the equivalence of the Rietveld method and the correlated integrated
intensities method in powder diffraction"
W. I. F. David
If I remember well; to get the corresponding Rfactor you just take the
correlated intensities chi^2 for your model and divide by the same
statistic with all I_calc set to zero and perhaps convert to a
percentage. (Hopefully Bill is reading and will correct that if I've
screwed it up).
Fortunately any statistics which rely on peak intensities are open to
abuse by using the wrong cell or space group etc, hence the value of
Rietveld Rfactors and a really high background ;-)
Best,
Jon
Olga Smirnova wrote:
Dear All,
How is life with conventional R factors when you always have to divide
by zero background?
Let's have time. Considering a part of the profile without peaks one
gets 100% cR.
I did not give the agreement factor; I would say those cR with all
non-excluded points is incorrect, but cR for
points with Bragg contribution is almost the same!
Do you decrease the Rs by adding the background or do you increase cRs
by subtracting the background?
OS
PS I did not ask my supervisor before sending such a mail.
