Dear Axel
Thanks for further clarification. Yes you are right in the sense that particles are very packed, and thus enlarging the box would thus be the only way out.
Regarding velocities, they are crucial in my case in the sense that the particles are in rigid contact, and there is basically no potential energy except from gravity, which is itself not significant. Am I misreading your point in any way?
Best,
Mohammad E. Torki