Prithwish,
The error is telling you what is missing. You do not have an integrator defined such as fix nve or fix npt. Therefore the atoms do not move from their intial positions as the error states.
Bill Evans
Postdoctoral Research Associate
Rensselaer Polytechnic Inst.
Nanotechnology Center MRC 275A
110 8th St.
Troy, NY 12180