[Chimera-users] Centro-symmetric space groups

[Thomas Goddard]

Hi Mike,

   Oops. The symmetries that invert coordinates don't work in the Unit 
Cell tool.  This is because it sets the transformation matrix (rotation 
and shift) for the asymmetric unit copies but it can only handle proper 
rotations.  Chimera is generally used for proteins which are handed (not 
symmetric under inversion) so such space groups generally don't occur 
for proteins.  But of course for small molecules they can.

   I'll fix this soon, maybe later this week.  I'll have to change atom 
coordinates instead of setting the transform matrix for the asymmetric 
unit copies.

	Tom


-------- Original Message --------
Subject: Re: [Chimera-users] Centro-symmetric space groups
From: Thomas Goddard <goddard at cgl.ucsf.edu>
To: Michael Day <mikeday at caltech.edu>
Date: 8/31/09 5:25 PM

> Hi Mike,
> 
>    If there is a bug I want to fix it.  Give me the example PDB and the 
> correct answer, say from some other software -- best if it were another 
> PDB file with the other asymmetric units in the unit cell.  If that is 
> difficult and image might suffice.
> 
>    I'm not clear on what you are saying is wrong.  Do you mean the 
> placement of the asymmetric units produced by the Unit Cell dialog for 
> space group "P -1" is wrong?  That would be pretty odd -- I'd expect 
> many space groups to give wrong results if there was a bug.  If that is 
> the issue, you may be getting tripped up by the fact that the asymmetric 
> units are placed according to the symmetries but then moved so that the 
> center of each asymmetric unit being placed lies within the unit cell 
> box having origin at the location given in the dialog.  The purpose of 
> this is to get a packed unit cell.  Probably should have an option to 
> disable that behavior so you just get exactly the standard symmetry 
> placement.
> 
> 	Tom
> 
> 
> -------- Original Message --------
> Subject: Re: [Chimera-users] Centro-symmetric space groups
> From: Michael Day
> To: Tom Goddard <goddard at cgl.ucsf.edu>
> Date: 8/31/09 4:45 PM
> 
>  > Tom,
>  >
>  > Thanks for the info! Especially the speedy response! The PDB definitely
>  > allows centrosymmetric groups even though they never appear in proteins.
>  >
>  > Centrosymmetric space groups still get mangled. Your file appears to
>  > have the right symmetry operators. I ran into this on a P-1 structure
>  > that is a stack of planar molecules but when I expand the unit cell I
>  > get some thing completely different.
>  >
>  > I've tried the PDB file with other viewers and I can get the correct
>  > representation but I much prefer Chimera to everything else I've tried
>  > (there's really no comparison).
>  >
>  > It seems one of the the most straightforward options would be to start
>  > with CIF files because they have the symmetry operators in them and the
>  > coordinates are the actual crystallographic coordinates and therefore
>  > wouldn't need to be transformed/deorthogonalized.
>  >
>  > It may be a bug with your deorthogonalization routine.
>  >
>  > Cheers,
>  > Mike
>  >
> 
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