Thanks, Steve
In my simulation, we used the force field of ReaxFF to discribe the interaction of water moleculars, so we did not used SHAKE package (“fix shake”)
For the Q2, I know the ways that caculated position and velocity of center of mass by chunk (Layer), we can caculate the shock velocity by read the refernces (Reference: Lan He, Thomas D. Sewell, and Donald L. Thompson,Molecular dynamics simulations of shock waves in oriented nitromethane single crystals, THE JOURNAL OF CHEMICAL PHYSICS, 2011,134, 124506)