VMD-L Mailing List

From: Jan Saam (saam_at_charite.de)
Date: Thu Mar 23 2006 - 15:30:53 CST

Hi all,

I've noticed this behaviour in several cases, too. It seems to be a
limitation of the algorithm due to Kabsch 1976. For small sets of atoms
(~<100) the rotation sometimes is just wrong.

Does anybody know a better alignment algorithm?

Also it would be interesting to know if the same problem also occurs in
Pymol or other viewers. If it works in Pymol it would be interesting to
see what they do better.
I stumbled over a reference in the internet where someone implemented
the Kabsch algorithm as a Pymol script. I wonder if that one aligns your
molecules correctly?

IS THERE A PYMOL USER LISTENING?
Could you try it with Visvaldas example?

Regards,
Jan

PS.
That's the original paper:
*[Kabsch, 1976]* Kabsch, W. (1976). A solution for the best rotation to
relate two sets of vectors. /Acta. Crystal/, 32A:922-923.

Did anybody read the following paper? I don't have access to it, but it
might contain some information about our problem:
*[Kabsch, 1978]* Kabsch, W. (1978). A discussion of the solution for the
best rotation to related two sets of vectors. /Acta. Crystal/, 34A:827-828.

Visvaldas Kairys wrote:

>Hi all,
>
>I am trying to use VMD to compute RMSD between selected residues, and for
>practical purposes I superimpose only the atoms under interest.
>Below is an example of molecules (in pdb format) which cause problem.
>
>molecule 1:
>
>CRYST1 1.000 1.000 1.000 90.00 90.00 90.00 P 1 1
>ATOM 1 CA GLU A 1 -29.843 154.588 35.594 0.00 0.00
>ATOM 2 CA GLU A 2 -27.292 160.642 36.936 0.00 0.00
>ATOM 3 CA GLU A 3 -32.648 156.854 50.794 0.00 0.00
>END
>
>molecule 2:
>
>CRYST1 1.000 1.000 1.000 90.00 90.00 90.00 P 1 1
>ATOM 1 CA GLU A 1 44.084 6.298 12.425 0.00 0.00
>ATOM 2 CA GLU A 2 39.046 2.420 14.373 0.00 0.00
>ATOM 3 CA GLU A 3 46.929 -0.875 26.932 0.00 0.00
>END
>
>If I open RMSD calculator extension, unselect backbone button
>and type "all" for atom selection, then click "Align",
>the molecules do not become aligned (it's pretty obvious), and "RMSD"
>is quite high. If I keep clicking "Align" and subsequent "RMSD",
>the position of atoms and RMSD keep changing.
>
>If I simply duplicate each atom in both of molecules above,
>yielding 6 atoms in each, the alignment and RMSD calculation proceeds
>correctly.
>
>I wonder if there is some formatting problem in my molecules or it is a bug.
>
>I had similar errors with two molecules consisting of 2 atoms. I didn't look
>carefully at other sizes of molecules, but big molecules seem to be OK.
>
>I am running VMD 1.8.4a22 on AMD64 Linux machine.
>
>Your help will be appreciated!
>
>Sincerely,
>
>Visvaldas Kairys
>University of Madeira
>Portugal
>
>
>
>
> 

-- 
---------------------------
Jan Saam
Institute of Biochemistry
Charite Berlin
Monbijoustr. 2
10117 Berlin
Germany
+49 30 450-528-446
saam_at_charite.de