[Chimera-users] Visualizing major and minor axes of inertia

[Elaine Meng]

Hi Andrés,
from <http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/midas/measure.html#inertia>

"The vectors v1, v2, and v3 are the principal axes (longest to shortest). The lengths a, b, c are half-diameters along axes v1, v2, and v3, respectively.”

Major is just the longest one.  There isn’t a direct option to show as arrows, sorry, only as the ellipsoid.  You could make the ellipsoid transparent if that helps (e.g. select it and use command “transparency 75 sel”).  

If it is mainly the directions of the vectors you care about rather than the magnitudes, you can (with the atoms selected) instead use commands to define a disc that is the plane (first two principal axes) and then the normal to that plane (third axis):

define plane sel number 1
define axis p1

<http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/midas/define.html>
However, the plane disc size and axis length do not show the vector magnitude, only direction.

Another option, but more work, is that you can take the reported values from “measure inertia" and calculate arrow endpoints and make a BILD text file describing those arrows and open it in Chimera.
<http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/bild.html>

You can open the example file linked to that page XYZ-axes.bild (just use File… Open) to show the X,Y,Z axes as arrows.

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Sep 3, 2018, at 3:26 AM, Felipe Vasquez <anfelvas at gmail.com> wrote:
> 
> Dear all,
> I am interested in visualizing the major and minor axes of inertia of a ligand in a PDB structure by using UCSF Chimera. Once I calculate the axis of inertia by entering in command line:
> 
> measure inertia sel
> 
> I obtained the following info (in reply log):
> 
> v1 =  0.985 -0.173 -0.013   a =  6.079   r1 =  2.103
> 	v2 = -0.086 -0.553  0.829   b =  4.564   r2 =  2.765
> 	v3 = -0.150 -0.815 -0.559   c =  1.127   r3 =  3.400
> 	center =   38.682   56.595   62.547
> 
> However, I do not still know:
> 
> 1- What are the major and minor axes of inertia? This info is given by the "a/b/c" or "r1/r2/r3" values?
> 2-How can I visualize the major and minor axes of inertia as planes (or arrows)?
> 
> Thanks in advances for your help.
> Best regards,
> Andrés Felipe Vásquez J., BSc, MSc.