how to apply constant velocity correctly

Hi dear all
I am performing nanoindentation using spharical diamond indentor taken as rigid body using morse potential. I am not using fix indent command.

I am using command
velocity dot set 0.0 0.0 -0.1 units box

for moving the indenter with the constant velocity. the problem is that dot move toward the substrate hit the substrate and then rebound after some time causes the force between indenter and substrate to becomes negative. I am unable to understand the problem the force is computed using
compute force dot group/group upper
I am waiting for your kind suggestions. any help and suggestion is appreciated

Muhammad Imran

The velocity command sets the initial velocity
of the atoms. If you want them to continue with
this velocity during a run, then you should set
the force on them to 0.0 via the fix setforce command,
else their velocity will be altered as they interact.