Dear Dr. Axel Kohlmeyer,
Thank you very much for your prompt reply.
actually, I want the anion to reach the end of the cell at the same time as the cation under the electric field.
Because its mass is 20 times that of a cation, I assumed the velocity of the anion to be 20 times that of the cation(all molecules of system except anion), but nothing changed compared to the case where the velocity was the same for the whole system.
the problem is that you did read the documentation for the velocity command well enough. the way you use it you do assign higher velocities to individual atoms in the one group compared to the other. but that does not mean that those groups have a net velocity as a group. in fact, with the “mom yes” you do remove any of such center of mass velocities.
As you said, by defining the velocity of the groups in the dump, I will check their velocities.
which should confirm what I just said. to assign a net velocity to the group you need to use the velocity “set” command. and you may want to do this with the “add” option so you add a center of mass velocity to a group of atoms that is otherwise equilibrated to a given temperature.
for setups like this, you also have to watch out for using a thermostat as that will affect such velocities significantly and in particular since they will significantly increase the global temperature. the thermostat will then cool down all motions and you will get a system very difference from what you intend to simulate. LAMMPS has all kinds of provisions for those cases, but they need to be used properly and that requires careful studying of the documentation.