# Properly including bromide in GROMOS96 54a7

**URL:** https://gromacs.bioexcel.eu/t/properly-including-bromide-in-gromos96-54a7/4836
**Category:** User discussions
**Tags:** forcefield
**Created:** [September 22, 2022, 1:38pm UTC](https://gromacs.bioexcel.eu/t/properly-including-bromide-in-gromos96-54a7/4836 "2022-09-22T13:38:25Z")
**Posts on this page:** 1
**Showing post:** 3

<div class="post-metadata">

### Author: ![alevilla](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/alevilla/32/439_2.png) [@alevilla](https://gromacs.bioexcel.eu/u/alevilla)
#### Post date: [September 30, 2022, 11:39am UTC](https://gromacs.bioexcel.eu/t/properly-including-bromide-in-gromos96-54a7/4836/3 "2022-09-30T11:39:23Z")

</div>

Hi,

> [@esmee](#):
>
> When I then tried to run the energy minimization step (following the Lysozyme tutorial) i got the error message “atomtype BR- not found”.

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 [https://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html](https://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html)

> [@esmee](#):
>
> In atomtypes.atp I can see that there is bromine (non-ionic) present, which is denoted BR. **Does this mean that when putting BR instead of BR- under “at type” in ions.itp, the parameters in e.g. ffnonbonded.itp for the neutral bromine are used also for the bromide**?

yes.

> [@esmee](#):
>
> If so, is this an ok approach anyway (I can for example see that values in ffnonbonded.itp differ between entries for CL and CL-, which is my reason for doubting the approach)?

BR and BR- are different, it is more appropriate to describe them using different LJ parameters.

> [@esmee](#):
>
> If it is not an ok practice, how do I properly add BR-? Do I have to add it also to atomtypes.atp, and to add values for c6, c12 and nonbond\_params in ffnonbonded (where does one obtain such values for bromide, if so)? Are there any other files of the gromos54a7 force field that have to be modified as well?

When you add a new ion, it is good to add it also in ions.itp.

\Alessandra

---

_[View the full topic](https://gromacs.bioexcel.eu/t/properly-including-bromide-in-gromos96-54a7/4836)._