Hi

I'm trying to calculate ion diffusivity in a plymer under electric field. However, I find that the electric field causes the entire system to drift in space. I attach the file of final and initial structure snapshots. A paper reproted that 'the momentum gain accumulates due to the periodic boundaries used—the phonon leaves one side enters the system from the other with no counteracting forces, which causes the entire system to drift in space'. How can i solve this problem?

I would be really grateful if you could help me.
Thanks