I am simulating a nanoparticle (group of atoms in a bcc lattice) in a polymer matrix. Currently, I am annealing my system down to T=0.2 and using fix aveforce command to average out the force on the beads of the nanoparticle. I set the initial velocity to 0 for all the beads and expecting same motion of all of its beads during the simulation run (as all the beads of the NP will feel the same force).
However, I was tracking down the distance between 2 of its beads and I found that it is decreasing continously (please see attached file). I am wondering why the atoms are moving to a different degree if they are feeling the same force.
I would also like to mention that I tried annealing the system using fix rigid command. It worked well till T=0.3. However on further lowering the temperature, I observed huge fluctuation in the pressure values and temperature values ( thats why I switched to fix aveforce). (please see the figures for the reference)
I would appreciate any help with fix aveforce or fix rigid.