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Dear Nicholas,
for a data set with 5132 unique reflections you should flag 10.5% for
Rfree, otherwise you could as well drop Rfree completely and use the
whole data set for refinement. At least this is how I understand Axel
Brunger's article about Rfree where he states that one needs 500-1000
reflections for a significant meaning of Rfree.
I have wondered where the '5%-rule' came in which compromises the Rfree
for low resolution data sets (especially with high symmetry).
If Axel Brunger's initial statement has become obsolete I would
appreciate some clarification on the required number of flagged
reflection, but until then I will keep on flagging 500-1000 reflections,
rather than 5%.
Tim
On 10/15/2011 10:48 AM, Nicholas M Glykos wrote:
>>> For structures with a small number of reflections, the statistical
>>> noise in the 5% sets can be very significant indeed. We have seen
>>> differences between Rfree values obtained from different sets reaching
>>> up to 4%.
>>
>> This is very intriguing indeed! Is there something specific in these
>> structures that Rfree differences depending on the set used reach 4%?
>> NCS? Or the 5% set having less than ~1000-1500 reflections?
>
> Tassos, by your standards, these structures should have been described as
> 'tiny' and not small ... ;-) [Yes, significantly less than 1000. In one
> case the _total_ number of reflections was 5132 reflections (which were,
> nevertheless, slowly and meticulously measured by a CAD4 one-by-one. These
> were the days ... :-)) ].
>
>
>
>
- --
- --
Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen
GPG Key ID = A46BEE1A
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