Disintegration of Solid Surface During Simulation

Dear All,

I want to model a lubricant between two Aluminium solid surfaces. Therefore, I used 3 different regions and assigned 3 different groups to each region.
For fixing the two above and bottom Al surfaces, I used the following command:

fix freezetop AlumTop setforce 0.0 0.0 0.0
fix freezebot AlumBot setforce 0.0 0.0 0.0

But after a few steps of NVT run, these two surfaces have disintegrated and I think it is due to fixing setforce command.

Is there any other way to fix these surfaces to make them not move along the z direction?

This is probably because the velocities are not 0.

Use (in addition to the setforces):

velocity AlumTop set 0 0 0
velocity AlumBot set 0 0 0

If you don’t want some atoms to move at all just exclude them from the group used by fix nvt.

Many thanks for your comments. I will try it.