FEP: ligand topology files

GROMACS version: 2019-20
GROMACS modification: No
Dear Gromacs users,
I am not able to find any tutorial describing how to write topology files to calculate the binding free energy due to functional groups swapping between ligands in a common binding site. I just found how to calculate the binding free energy of solvation.
Could you please share some links?
Thank you in advance

It is not a tutorial, but the pages below may help

see “Topologies for free energy calculations” in

see free energy calculation in

Best regards