# Adding ion to a force field
**URL:** https://gromacs.bioexcel.eu/t/adding-ion-to-a-force-field/4918
**Category:** User discussions
**Created:** [September 29, 2022, 12:31pm UTC](https://gromacs.bioexcel.eu/t/adding-ion-to-a-force-field/4918 "2022-09-29T12:31:34Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@esmee](https://gromacs.bioexcel.eu/u/esmee)
#### Post date: [September 29, 2022, 12:31pm UTC](https://gromacs.bioexcel.eu/t/adding-ion-to-a-force-field/4918/1 "2022-09-29T12:31:34Z")
GROMACS version: 2021.3
GROMACS modification: Yes
Dear GROMACS users,
I would like to add an ion to a force field. The residue for the neutral atom corresponding to the ion is already present in the force field. Which files have to be modified and how?
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### Author:  [@alevilla](https://gromacs.bioexcel.eu/u/alevilla)
#### Post date: [September 30, 2022, 11:41am UTC](https://gromacs.bioexcel.eu/t/adding-ion-to-a-force-field/4918/2 "2022-09-30T11:41:05Z")
Hi,
See my answer to your previous post. I hope it cover all the points
> [@Properly including bromide in GROMOS96 54a7](https://gromacs.bioexcel.eu/t/properly-including-bromide-in-gromos96-54a7/4836/3):
>
> Hi, The reason is that you change the name from BR → BR- and BR- is not defined as atomtype in the force field files. Atom type information are storage in ffnonbonded.itp in the [atomtypes] directive and [nonbond\_params] directive (for gromos force field) and in the database atomtypes.atp. To know more on the format see [File formats — GROMACS 2022.3 documentation](https://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html) yes. BR and BR- are different, it is more appropriate to describe them using different LJ parameters. When you add a…
\Alessandra