[ccp4bb]: indexing & twinning

[<ernst@utbc08.cm.utexas.edu>]

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I am a bit perturbed by this ambiguity of unit cell choice between
multiple data sets collected from one crystal (and presumably not reoriented).

This is a problem that has been discussed on this bb and EJD and friends
have produced a tutorial about the geometrical ambiguities of several
space groups.  So, we should all be forwarned and aprehensive when
indexing (I will refer to denzo but similar things exist for DPS and others
auto indexers).

In the particular case of one xtal mult. data sets the unit cell
and the orientation matrix can be enforced in denzo. denzo
has for example the "print zones" command that will print all
of the symmetery related orientations (doesn't help for hexagonal
abiguity) and the exact lines needed to feed to denzo a particular
set of refined parameters is found in the last 10-20 lines of
the denzo output file (tail -20 lys_a2_001.x > or.par)
the two crucial lines are "unit cell" and "crystal rotx roty rotz"
less crucial are "x beam y beam" and "distance".  simply
comment out the peaks file line [ peak search file xxxxxx.yy] and
insert the lines from the denzo output file.

If there is crystal movment of some sort, index , print zones
and compare the rot xyz values to the first set (for hex cell
be aware that rotz may need +- n*30 degrees to achieve equivalence)

If this seems to be whipping a dead horse, my only excuse is that
this is experience speaking.  In a mad,semet, long distance data collection
attempt data sets for 4 different wavelengths were collected on
a p3(2)21 crystal. each data set was indexed a priori and some of
the sets were indexed with one seting some with the other  then
scalepacked, solve'd, resolve'd , maps generated, examined and
pronounced "promising".
                                                                                                    And, of course, the agreement between the data sets checked
at scaling time (not an easy feat with solve)

Steve Ernst