ABFE(absolute binding free energy charge correction)

GROMACS version:
GROMACS modification: Yes/No
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My ABFE calculation involves annihilation of a positive charged ligand (+1). I would like to add a dummy ion whose charge will change from 0 to +1 during the annihilation to prevent the alteration of the net charge. I have no idea how to change the state of two molecules at the same time. Could anybody give suggestions?