Rigid vs. immobile

Dear LAMMPS Users and Developers,
I already have used the combination of commands:
setforce 0 0 0 and valocity 0 0 0 to make a group both Rigid & Immobile.
However, how is the way to make a group RIGID but MOBILE at the same time.
I mean by MOBILE not moving by imposing “velocity” instead by interacting with other groups of the model.
Thank you in advance.
Rouhollah.

Dear LAMMPS Users and Developers,
I already have used the combination of commands:
setforce 0 0 0 and valocity 0 0 0 to make a group both Rigid & Immobile.
However, how is the way to make a group RIGID but MOBILE at the same time.
I mean by MOBILE not moving by imposing "velocity" instead by interacting
with other groups of the model.

http://lammps.sandia.gov/doc/fix_rigid.html