Adding Velocities From Python via fix python/invoke

Hi All,
I am trying to write a python script which runs LAMMPS as a library with python/lammps interface

from lammps import lammps

During the simulation lets say every 100 steps, I would like to add Velocities to some atoms in the system just before the verlet step.
I am trying to simulate electron scattering and momentum transfer to atoms.
I am using

fix pf  all python/invoke 50 post_force post_force_callback

So this fix will call a function in my python script and I can manipulate forces or velocities of the atom.

So my question is should I just update velocities at this step or should I update forces to add the momentum via

Delta P = Delta F * timestep


Dundar Yilmaz, Ph. D.
Associate Research Professor
Department of Mechanical and Nuclear Engineering
138 Research East Building
Penn State University
University Park, PA 16802

For post_force you should increment the forces.
The Verlet integration within LAMMPS will use
those forces to increment velocities.