Thanks a lot Aidan! I didnt know the fact that each rigid body had 6 dof.
But I still have 2nd question which I still cant understand.
Including the fact that rigid body has 6 dof, 2nd question still looks so
weird.
I just computed the ke and temp of H2O, but dof is still related to the
rigid bodies.
(I tested another system composed of 250 water and 10 ions, however in this
case, dof of water was not related with the dof of ions.)