[gmx-users] principal moments of ellipsoid

[Mark Abraham]

On 10/05/2011 8:23 PM, lammps lammps wrote:
> Hi,
> I want to study the shape change of a macromolecule which seems like a 
> ellipsoid.  So, I need obtain the three principal moments of the 
> molecules.
> Is the command of g_gyrate helpful for me? The sum of the three 
> principal moments should be equal to the <Rg^2>.  However, I used the 
> command g_gyrate -f traj.gro -s nvt.tpr -n anly.ndx -o shape.xvg -p 
> obtain the Rg and other three qualities.
> It seems that the sum of the last three qualities is not equal to the 
> Rg^2. Is there something wrong?

Why should that sum equal Rg^2?

Mark