[gmx-users] Frozen group moves!

[Alex]

Dear all,
I freeze a group in my system in all directions as normal:
freezegrps               = GR
freezedim                = Y Y Y
So, I was expecting that the coordinates and velocities do not get updated
during the simulation, however, here is just part of in.gro and out.gro in
which the coordinates and velocity components have changed.
    #Y          #Z       #Vx     #Vy       #Vz
  #Y          #Z       #Vx     #Vy       #Vz
  0.271   0.049  0.0000  0.0000  0.0000                        |  0.271
 0.050  0.0000  0.0000 -0.0637
  0.249   0.020  0.0000  0.0000  0.0000                        |  0.249
 0.020  0.0000  0.0000  0.1823
  0.251   0.490  0.0000  0.0000  0.0000                        |  0.251
 0.490  0.0000  0.0000 -0.6805
  0.228   0.524  0.0000  0.0000  0.0000                        |  0.228
 0.524  0.0000  0.0000 -0.1568
  0.251   0.055  0.0000  0.0000  0.0000                        |  0.251
 0.055  0.0000  0.0000 -0.4114
  0.369  17.054  0.0000  0.0000  0.0000                        |  0.369
17.055  0.0000  0.0000 -0.5276
  0.390   0.272  0.0000  0.0000  0.0000                        |  0.390
 0.273  0.0000  0.0000 -0.2386
  0.184   0.444  0.0000  0.0000  0.0000                        |  0.184
 0.444  0.0000  0.0000 -0.1694
  0.365   0.444  0.0000  0.0000  0.0000                        |  0.365
 0.444  0.0000  0.0000  0.1299
  0.361   0.553  0.0000  0.0000  0.0000                        |  0.361
 0.554  0.0000  0.0000 -0.5277

Anybody knows what might be the reason and how one can avoid that to happen?

Regards,
Alex