Hi

I'd run the reflection file through pointless rather than rely on  
cell dimensions and/or systematic absences. This is a quick and easy  
test to do and more reliable. As Bert suggests, the only real way to  
know the symmetry is after successful structure solution and  
refinement (but even then you can be fooled...).

On 8 Jul 2011, at 21:07, Van Den Berg, Bert wrote:

> I'd say its very likely to be orthorhombic. Refinement should tell  
> you.....its the best way to determine the space group anyway. Why  
> do you doubt its orthorhombic? Is Vm reasonable?
> It could be monoclinic and merohedrally winned with the beta angle  
> very close to 90 degrees, but my money is on orthorhombic. if  
> refinement fails I would try monoclinic plus/minus twinning. As for  
> the operators, xxxxxx.triage will tell you and xxxxxx.refine will  
> apply them for you during refinement....;-)
>
> Bert
> ________________________________________
> From: CCP4 bulletin board [[log in to unmask]] On Behalf Of Raji  
> Edayathumangalam [[log in to unmask]]
> Sent: Friday, July 08, 2011 3:24 PM
> To: [log in to unmask]
> Subject: Re: [ccp4bb] Potential Space Group Issue
>
> Oops sorry for the slippery fingers. I meant h00, 0k0 and 00l in my  
> original email and NOT "00h, 00k, 00l". Note the correction  
> especially if you are a first-year graduate student trying to learn  
> stuff from these emails :)
>
> Raji
>
>
>
> On Thu, Jul 7, 2011 at 11:30 PM, Raji Edayathumangalam  
> <[log in to unmask]<mailto:[log in to unmask]>> wrote:
> Hello Everyone,
>
> I have a 3.1 Ang dataset for which I'd like to get to the bottom of  
> what the correct space group is.
>
> The current unit cell in p212121 is 98.123   101.095   211.201     
> 90.000    90.000    90.000
> I fed the reflection data into Xtriage to look for twinning and  
> pseudotranslational NCS and there is no indication for either issue  
> in the Xtriage output. Also, all odd 00h, 00k, 00l reflections are  
> systematically absent as they should be for p212121.
>
> However, my colleague who is also working on the same dataset  
> recently reprocessed the data in P21. Here's the cell in p21:
> 98.010  100.940  210.470  90.00  90.04  90.00 p21
>
> I am not sure if BETA=90.04 is significant enough to treat as p21  
> (0.04% deviation of beta angle from ideal lattice for p212121). I  
> don't think so but I could be wrong. Could someone please clarify?
>
> Also, what kind of twinning and twinning operators can relate a  
> p212121 cell to a p21 cell with almost identical unit cell  
> parameters as that of the p212121 cell and leave all systematic  
> absences intact?
>
> Thanks much.
> Raji
>
>
> -----------
> Raji Edayathumangalam
> Instructor in Neurology, Harvard Medical School
> Research Associate, Brigham and Women's Hospital
> Visiting Research Scholar, Brandeis University
>
>
>
>
> --
>
> ----------------------
> Raji Edayathumangalam
> Instructor in Neurology, Harvard Medical School
> Research Associate, Brigham and Women's Hospital
> Visiting Research Scholar, Brandeis University

Harry
--
Dr Harry Powell,
MRC Laboratory of Molecular Biology,
Hills Road,
Cambridge,
CB2 0QH