Hi Kevin, I would say that the most useful place for this token will be in reduced-data land prior to getting phases - so for instance going into Phaser it could make a sensible choice about what spacegroups to test based on the accuracy information, likewise experimental phasing / dm programs. Once these programs have computed phases they will (most likely) be in a position to decide the correct spacegroup and write this out with some authority along with the phases.
So what I am getting at is that you are unlikely to have phases without knowing what the right spacegroup is. Certainly, you are unlikely to have correct phases ;o) This cannot possibly "cure" the spacegroup naming problem, but it could certainly allow people to say what they mean when assigning some spacegroup information to a reflection file... Cheers, Graeme -----Original Message----- From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On Behalf Of Kevin Cowtan Sent: 09 November 2007 14:12 To: [email protected] Subject: Re: [ccp4bb] pointless (1.2.0) and enantiomorphic SG's This is great! But before we all get carried away, a quick reality check is in order. This isn't an area I've thought about, but my limited understanding raises the following issues.... Given that an MTZ file contains a cell, an indexing of the data, and possibly phases involving a choice of origin, the use of an MTZ file immediately implies that some decision has been made about the spacegroup. Selecting a different spacegroup from a list of possibilities may involve reindexing, and/or changing origin/enantiomorph dependent properties such as phases and HLs. It would be possible for an MTZ file to say something like this: 'This file is generated assuming spacegroup X. Spacegroups X1, X2, X3, X4, X5.... are also possible.'. It should also be possible to compute on the fly whether a particular column type needs to be modified for a different spacegroup. How will we present this in user interfaces? As a starting point, until a spacegroup has been settled upon, the user will need the option of picking the spacegroup after having selected the MTZ. This means potentially a reindexing step added to every single task - messy. As an intermediate, being able to change the 'currently selected' spacegroup as a separate task would be an improvement. Phil Evans wrote: > I agree > > On 9 Nov 2007, at 13:50, Eleanor Dodson wrote: > >> As I often say!!! The mtz format should carry point group and >> alternate SGs - then be upgraded when youknow the correct SG.. >> >> Eleanor >> >> Phil Evans wrote: >>> It just picks the first in the list, to store in the output MTZ >>> file, which can only handle one (92 < 96) Phil >>> >>> >>> On 8 Nov 2007, at 22:26, Bryan W. Lepore wrote: >>> >>>> when pointless (1.2.0) finds enantiomorphic SG's, what is the >>>> criterion for 'Selecting' one over the other? >>>> >>>> e.g. i ran pointless on some tetragonal data, and the enantiomorphs >>>> SG92/SG96 are selected as strong candidates >>>> >>>> Spacegroup TotProb SysAbsProb Reindex Conditions >>>> <P 41 21 2> ( 92) 0.956 0.956 00l: >>>> l=4n, h00: h=2n (zones 1,2) >>>> <P 43 21 2> ( 96) 0.956 0.956 00l: >>>> l=4n, h00: h=2n (zones 1,2) >>>> >>>> ... then pointless reports : >>>> >>>> Selecting space group P 41 21 2 as solutions are enantiomorphic >>>> Best Solution space group P 41 21 2 >>>> >>>> ... is that b/c pointless can only report one, and SG92 came up first? >>>> >>>> -bryan >>> >>> >> > >
