[gmx-users] Distance between COM of two groups

[Sajjad Kavyani]

Dear experts,

I am puling group2 toward group1 and I want to calculate the distance
between COM of two groups versus time.
For the g_dist it was a straightforward process with the selection menu.
But for the "gmx distance" I used:

gmx distance  -f test.xtc -s test.tpr -oav -oxyz -n index.ndx
-select 'com of group "group1" plus com of group "group2"'

But both outputs distances are a growing value over time (!!)
(actually the average value is the same with sqrt(x^2+y^2+z^2)) where
the distance meant to be decrease over time!

Could you please help me with the commands?