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William Scott wrote:
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Once you pick the crystal setting you use to index all subsequent
choices will be correct for that setting. E.g. your molecular
replacement or phasing will give the results for the choosen setting
and CNS will likewise create Fcalcs that match the choice you made to
index your data. So only when you have to merge batches of data that
have been independently indexed do you run the risk that the
individual batches do not all match up. Once you get that step right
everything else falls in step. Phil's new program should be a big help
here.
Bart
Hi Bart:
Sorry if I am being clueless (which is hardly unusual), but how would a
data processing program know in advance which way (eg) CNS would choose
h and k for purposes of calculating Fcalcs from say a PDB file placed in
a trigonal cell (by whatever means)? Presumably the choice that (eg)
CNS makes is an arbitrary one, but even if it were not arbitrary, how
can a data processing program know this in advance?
Bill
CNS doesn't have to choose anything. You provide it with a structural
model in a predefined unit cell. Assuming the positioning in the unit
cell corresponds to the choice of structure factor indexing, the Fobs
and Fcalc will match. The position of the model in the cell was
determined using the observed data (by either experimental phasing or
molecular replacement) and therefore the position will again match
whatever setting your data processing program choose to use. So what I'm
trying to say is that the indexing of Fobs defines one of several
possible settings and the structure solution procedure will
automatically match whatever setting that is because it finds the
correct solution relative to the choosen setting.
I hope that helps. If not, try coffee or something a bit stronger :)
Bart
--
==============================================================================
Bart Hazes (Assistant Professor)
Dept. of Medical Microbiology & Immunology
University of Alberta
1-15 Medical Sciences Building
Edmonton, Alberta
Canada, T6G 2H7
phone: 1-780-492-0042
fax: 1-780-492-7521
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