[Chimera-users] Principal axis of inertia

[A. Di Fenza]

Dear Elaine,

your guess was perfectly right! Thank you very much for the detailed 
instructions it is the first time I use Chimera and that was what I 
needed, now it works.

I have a question about the algorithm. As far as I have understood, the 
script looks at the element name, say CA or  N or C, to assign the atom 
mass. So that if I have a pdb made of CA atoms only, the atom masses 
used in the inertia matrix calculation are all the same. Is this right?
I need to be sure because I calculated with your script the inertia 
matrix for the same protein first with all-atom and then with only the 
CA, and I got that they are very similar although using the same mass 
for all the atoms is a quite big approximation.

Thank you very much for your help!
Armida


Elaine Meng wrote:

> Hi Armida,
> I just tried the script in Chimera 1.3 and in a recent daily build  
> (1.4) and it worked in both.
>
> I'm guessing that when you tried it, Chimera did not know the file  
> type of the script because it was not named ending with ".py".  If 
> you  look at the link below, even though the mail program names the 
> file  "attachment.ksh", the original name is "inertiapdb.py".  You 
> could  name it that or actually anything that ends in .py and then it 
> should  work.
>
> For example, I saved the script file to my Desktop and named it  
> inertiapdb.py.  Then I started Chimera, then opened a PDB file, then  
> used "File... Open" in the menu to open inertiapdb.py -- an ellipsoid  
> is shown in the Chimera window and numerical information is shown in  
> the Reply Log.
>
> I hope this helps,
> -----
> Elaine C. Meng, Ph.D.                          meng at cgl.ucsf.edu
> UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab
> Department of Pharmaceutical Chemistry
> University of California, San Francisco
>                      http://www.cgl.ucsf.edu/home/meng/index.html
>
> On May 14, 2009, at 6:42 AM, A. Di Fenza wrote:
>
>> Hi Tom,
>> I saw your message on the web and I tried to use your script on  
>> Chimera 1.3.
>> Unfortunately it does not work, it give me the following message:
>>
>> 0 models opened
>>
>> Do you think it is because of the Chimera version? Do you have  
>> suggestion?
>>
>> Thank you very much,
>> Armida
>>
>> Oops.  That script I posted for calculating the inertia of a molecule
>> had some problems.  The printed matrix of axes was transposed though  
>> the
>> correct axes were used for the displayed ellipsoid.  Also I was using
>> inertia matrix I_ij = sum_over_k(m_k*r_ki*r_kj) omitting the diagonal
>> term.  Should have been I_ij = sum_over_k(m_k*(|r_k| - r_ki*r_kj)).   
>> And
>> finally I incorrectly computed the ellipsoid size along each axis.
>> Everything was a little bit wrong.  Here's the fixed version.   
>> Tested in
>> Chimera version 1.2500 but should work with earlier versions.
>>
>>    Tom
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