On Friday 18 January 2008 09:30:06 am Ethan A Merritt wrote:
>
> Rmerge is an average over replicate measurements of the intensity for
> identical [hkl]. Rsym is an average over the measurements for all symmetry
> equivalent reflections.
>
> In the presence of anomalous scattering, Rsym will be higher than Rmerge
> because the Bijvoet pairs, although symmetry related, do not have identical
> intensities.
>
> One might logically report two values for Rsym, one which averages
> over the Bijvoet-paired reflections and one which does not.
>
This has been an eye-opening discussion for me. I've been really surprised
that there's been such a diversity of opinion about what these common
terms ought to refer to, and the fact that my understanding was wrong.
I always thought that Rsym was an average over all symmetry equivalent
reflections from the same crystal (including Bijvoet pairs) and Rmerge was
properly restricted to cases of multi-crystal averaging. (My versions of
Table 1's from single crystals have used "Rsym" rather than "Rmerge".)
I wonder if the problem here is that the terms have become overloaded (and
hence non-specific). In that sense "Rmerge" is a particularly unfortunate
name as every R that we're discussing is a really a merge of some sort or
another. (In the most naive sense, "Rmerge" might be thought to be the R
for whatever variation of reflection merging the experimenter chooses to do.)
One possible solution would be to push the community towards a new set of
terms with clearly defined meanings (and whose names would be used
explicitly by new releases of MOSFLM, HKL2000, etc. and changes for
new entries in the PDB).
If new terms were to be adopted, they ought to specifically distinguish
between single crystal and multi-crystal merging. I see three such
R values that might be useful (I've arbitrarily chosen names to distinguish
them from each other and the older terms):
Rhkl - R of identical hkl's
Rrot - R of symmetry-related hkls, but not Bijvoet pairs
("rot" coming from the concept that all symmetry-related
reflections can be found via rotations in reciprocal space and
the fact that "sym" has already been used)
RBijvoet - R of symmetry-related and Bijvoet-related hkls
(including reflections related by both rotations and an inversion
center in reciprocal space)
Rhkl,multi - multi-crystal version of Rhkl
Rrot,multi - muti-crystal version of Rrot
RBijvoet,multi - multi-crystal version of RBijvoet
The downside of adopting new names is that it makes the previous literature
obsolete, but I wonder if the older terms were ambiguous enough that that's
not such a problem.
--
Christopher Putnam, Ph.D.
Assistant Investigator
Ludwig Institute For Cancer Research
