To allow atoms to move in particular direction

Dear all,

I want to allow a group of atoms to move only in z-direction.
So, I used fix/setforce command to zero the forces in x,y direction like this,

fix 4 upper setforce 0.0 0.0 NULL

However, the group translates in y direction during simulation.
In next trial, I used fix/momentum command to zero momentum in x,y direction.

#fix 4 upper setforce 0.0 0.0 NULL
fix 4 upper momentum 1 linear 1 1 0 angular

Unfortunately, it still moves in y direction.
Would anyone please help me to solve this problem?



fix lineforce ???

Your atoms probably have initial velocity in the y direction.
Zero it out via the velocity set command.


Dear Steve,

Thank you for your response.
However, it seems velocity set command does not work.
I zeroed the initial velocity in x,y direction by velocity set command as following,

velocity upper set 0.0 0.0 NULL units box
fix 3 upper rigid single torque * off off off
fix 4 upper setforce 0.0 0.0 NULL

The model still translates in y-direction during simulation (approx. after 30 ps).

Is there any way to fix the model only to move in z-direction for entire simulation time?



If you turn off forces in y (with an initial velocity of 0.0)
atoms will not move it that dir. If they are, then you haven't
done it right.