How do I add nitrate anion for simulation with AOT reverse micelle?

GROMACS version: 2020.4
GROMACS modification: No

I would like to perform simulation of nitrate anion within CHARMM 27 force field parameters for a simulation of potassium nitrate within an AOT reverse micelle. I obtained the nitrate anion parameters from SwissParam ( and the included the .itp file in my topology file.

However, the nitrate.itp file’s atom types are not present in the force field. So, I added them to ffnonbonded.itp and atomtypes.atp files.

I ran a small simulation, and it appears that the parameters are being read in. However, I am not sure if what I have hacked through is appropriate. I was wondering if anyone else would be able to provide insight into this process and/or confirm that this is an acceptable way to add new atom types.

Thank you,