In your first post, you asked about the pitfalls of using tetramethylammonium as a starting point. These include:
- the nitrogen L-J parameters being totally shielded from the solvent by the methyl groups,
- methyl groups having totally different electrostatic/electronegativity properties than hydrogens,
- methyl groups having totally different bonded properties than hydrogens.
In summary, of all the protonated amines, tetramethyl ammonium is the worst possible starting point for assigning ammonium parameters. Rather, the staring point of choice would be methylammonium (RESI MAMM). You can just keep the same atom types and have 0 missing parameters; I strongly feel the existing methylammonium parameters are very suitable for ammonium. As for the charges, you can just set all 4 hydrogens to 0.33 (the value in MAMM), and adjust the nitrogen by -0.02 so that the total charge is +1. Finished!