[lammps-users] how to add an atom with some velocity run towards a relaxed sample and interact with the sample

Groups are static in LAMMPS even if they are intitially defined as atoms in
a region. One exception is that when fix deposit creates an atom it
will add it to the group you specify. It doesn't matter whether that group
is initially empty or has atoms in it, the new ones will be added. The group
does need to exist however, so making it initially empty is fine.