Another cause of difficulty - nothing really to do with the spacegroup
selection - is when one copy of the model has much higher B factors than
others. Most MR searches assume that the copies contribute more or less
equally to scattering.
If you assign too high a symmetry this will make MR less likely to work.
This usually happens when the data is twinned.
Eleanor
On 02/16/2011 05:57 PM, Edward A. Berry wrote:
I think the question was not concerning right vs wrong space
group, but right vs a lower symmetry superset space group -
P4322 vs P43 for example. I would also be interested in
the answer.
At first glance it appears harder in the lower symmetry
because with each monomer you would be searching for a smaller
fraction of the contents of the AU. But if the rotation
function doesn't consider space group symmetry (?)
does that matter?
eab
Vellieux Frederic wrote:
Ting-Wei Jiang wrote:
Dear experts,
Sorry for a simple question but confusing me so much!
Does it make bad effects on determining the number of identical
molecules in ASU by choosing low symmetry space group.
For example,If I choose lowest symmetry(p4) instead of higher
one(p43212).
Does it cause any trouble in determining structure while we try to
find the solution by molrep.
If molrep compare the patterson of observe and model in P1 space
group, it doesn't matter in choosing a lower s.g.?
Any suggestion will be appreciated. Thanks in advance.
Ting wei
For molecular replacement: it does not matter if you carry out the
rotation search in P422 or P43212. To understand this, ask youself the
following question: what is the symmetry of the Patterson compared to
that of the crystal? The symmetry of the Patterson is obtained from the
symmetry of the crystal by converting all translation operators to the
corresponding non-translation operators and adding a centre of symmetry.
Hence for the rotation function, the symmetry of the Patterson will be
P4/mmm for space group P422 and P4/mmm for P43212. Hence no difference
for the rotation function.
Different for P4 though: the symmetry of the Patterson there is P4/m.
Hence the rotation function can be computed once for all space groups
that have Patterson symmetry P4/mmm. No need to repeat the rotation
function calculations several times.
The situation is different for the translation function however. This is
the stage at which you distinguish between all these space groups
(including the pair of enantiomorphs P41212 and p43212). Failure to
assign the proper space group cannot give you a satisfactory model.
HTH,
Fred.
