You could just use "define axis" with "massWeighting true" -- as far as I know, it should give the same principal axis as "measure inertia" for the same set of atoms.
<
https://rbvi.ucsf.edu/chimerax/docs/user/commands/measure.html#inertia>
<
https://rbvi.ucsf.edu/chimerax/docs/user/commands/define.html#axis>
Both commands take specified atoms, so they are equally "biased," if by biased you mean based on the atoms you specified. You can specify all atoms in the model if that's what you want, but it would really include everything like waters, ligand, etc.
The "measure inertia" help explains the output (v1, v2, v3 etc.) and yes the first one is the longest one.
<
https://rbvi.ucsf.edu/chimerax/docs/user/commands/measure.html#inertia>
I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.
UCSF Chimera(X) team
Resource for Biocomputing, Visualization, and Informatics
Department of Pharmaceutical Chemistry
University of California, San Francisco
> On Feb 23, 2025, at 2:46 PM, junjie han <hanjunjie2014@gmail.com> wrote:
>
> Hi Elaine,
>
> Thanks for your great response. It is really helpful. Could I ask more questions?
> Based on your direction, I already figured out how to define the axes and planes and then use 'angle' command to measure the angles between them.
> However, it may induce a bit of a bias by defining the axes manually. Could the ChimeraX generate a more accurate axis automatically that is through the mass center of the molecule?
> For instance, the command 'measure inertia #1/A showEllipsoid true' can generate the ellipsoid that is oriented based on the principal axes of inertia. I assume that the principal axis of inertia is through the mass center of the molecule.<image.png>What
I need is to generate axes like the black and red lines that go through the center of mass of the molecule and divide it into two equal parts. So how do I get this?
>
> Below is the output after I ran the command 'measure inertia' to show the ellipsoid of two subunits of a complex.
>
> measure inertia #1/A showEllipsoid true
> Inertia axes for 8dzs, 2074 atoms
> v1 = -0.508 -0.554 0.659 a = 36.027 r1 = 12.001
> v2 = 0.356 -0.832 -0.425 b = 19.480 r2 = 18.103
> v3 = 0.784 0.019 0.620 c = 18.455 r3 = 18.316
> center = 125.9 126.14 150.23
>
> measure inertia #1/b showEllipsoid true
> Inertia axes for 8dzs, 1641 atoms
> v1 = 0.520 0.842 0.141 a = 39.590 r1 = 11.344
> v2 = -0.329 0.045 0.943 b = 18.123 r2 = 19.403
> v3 = 0.788 -0.537 0.301 c = 17.747 r3 = 19.472
> center = 148.04 146.61 117.1
>
> The output shows three vectors, v1, v2, and v3 and the mass center of the subunit. My question is whether the v1 vector represents the direction of the black line of subunit A. If it is, I can define the axis based on the two coordinates (0,0,0) and ( -0.508,-0.554,
0.659) for unit A, and the coordinates (0,0,0) and ( 0.520,0.842, 0.141) for subunit B. After this, I can use the command 'angle' to calculate the angles between two axes.
> Is my understanding correct? Please let me know if it is, or share your strategy when you have a chance.
> Thank you very much.
>
> Looking forward your response,
> Jianming
>
>
>
> From: Elaine Meng <meng@cgl.ucsf.edu>
> Sent: Sunday, February 23, 2025 10:44 AM
> To: junjie han <hanjunjie2014@gmail.com>
> Cc: chimerax-users@cgl.ucsf.edu <chimerax-users@cgl.ucsf.edu>
> Subject: Re: Measure angle between domains in chimeraX
> Hi Junjie,
> In general there are two ways:
>
> (1) use Define Axes/Planes/Centroids tool or "define" command to define axis or plane objects based on specified atoms. Then you can get the crossing angles between any pairs of these objects using the Axes/Planes/Centroids GUI or the "angle" command. See
the following for details:
> <
https://rbvi.ucsf.edu/chimerax/docs/user/tools/axesplanes.html>
> <
https://rbvi.ucsf.edu/chimerax/docs/user/commands/define.html>
> <
https://rbvi.ucsf.edu/chimerax/docs/user/commands/angle.html>
>
> (2) if the two domains are structurally similar to each other, you could open two copies of the structure and fit one onto the other, superimposing the parts you want to superimpose (using whatever method you want, "align" or "matchmaker" or manual movements).
Then you can get the relationship between the two models with "measure rotation" -- see the following for details:
> <
https://rbvi.ucsf.edu/chimerax/docs/user/commands/measure.html#rotation>
> <
https://rbvi.ucsf.edu/chimerax/docs/user/commands/align.html>
> <
https://rbvi.ucsf.edu/chimerax/docs/user/commands/matchmaker.html>
>
> I hope this helps,
> Elaine
> -----
> Elaine C. Meng, Ph.D.
> UCSF Chimera(X) team
> Resource for Biocomputing, Visualization, and Informatics
> Department of Pharmaceutical Chemistry
> University of California, San Francisco
>
>
> > On Feb 22, 2025, at 8:39 PM, junjie han <hanjunjie2014@gmail.com> wrote:
> >
> > Hi Elaine,
> >
> > I hope this email finds you well. May I ask how to measure the angle between two domains from one molecule or two different molecules in ChimeraX. Here are some examples below.
> > Example 1:
> >
> > <image.png>How to calculate these angles (54°, 38°)？
> >
> > Example 2:<image.png>How to calculate this angle (75°) ?
> >
> > Example 3:<image.png>How can I measure the rotation angle between A and B, or between two specific regions of these two molecules in ChimeraX?
> > If I need to calculate the rotation angles of two molecules relative to a specific molecule, how can I define a shared axis for calculating the rotation angles? The rotation axes are different when I use the command “measure rotation A toModel B” to calculate
the rotation angles, which is not an ideal way to compare them in parallel.
> >
> > Thank you in advance for your any suggestions.
> >
> > Very appreciated,
> > Junjie