How to apply electric field in input .mdp file?

GROMACS version: gmx_mpi, VERSION 5.1.4
GROMACS modification: Yes/No
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we applied constant electric field of 0.99 V/nm along Z-axis , but we did not observe any ions separation in the system.

Hi!
It would be good to have some more information to be able to help you more. Could you describe the system you are simulating? What is the purpose/goal of the simulation? What electric-field settings do you use in the mdp file?

Have you read the Electric fields - GROMACS 2024.3 documentation part of the manual?