# [gmx-users] Gyration tensor

Salman Zarrini salman.zarrini at gmail.com
Thu May 23 18:01:31 CEST 2019

```Calculate the radii of gyration about the principal axes.

Dear all,

The -p flag in gmx gyrate would calculate the radii of gyration about the
principal axes, Rga, Rgb and Rgc. I wonder what the relation between the
eigenvalues of diagonalized gyration tensor and what we get here in Gromacs
as Rga, Rgb, Rgc is?

Also, I expect to see a simple relation likes ``Rg**2 = Rga**2 + Rgb**2 +
Rgc**2'' or ``Rg**2 = Rga + Rgb + Rgc'' where Rg is the total radii of
gyration of the object given by gromacs. Nevertheless, neither of these
equations fits with what gmx gyrate -p outputs (below) in my simulations.
Time                 Rg                 Rga              Rgb
Rgc
...
11586.0     2.52262     1.39226     2.29820     2.34673
11586.3     2.52194     1.39341     2.29706      2.3457
11586.6     2.52328     1.39318     2.29834     2.34746
11586.9      2.5268     1.39268     2.30208     2.35167
11587.2     2.52737     1.39083     2.30287      2.3532
11587.5     2.52599     1.38826     2.30281     2.35181
11587.8     2.52638     1.38821     2.30404     2.35148
11588.1     2.53102     1.39325     2.30731     2.35527
...
Would you please let me know what I am doing wrong?

Regards
Salman
```