# How to add salt？

**URL:** <https://gromacs.bioexcel.eu/t/how-to-add-salt/13644>\
**Category:** User discussions\
**Created:** [August 5, 2026, 12:57am UTC](https://gromacs.bioexcel.eu/t/how-to-add-salt/13644 "2026-08-05T00:57:10Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ZXY](https://avatars.discourse-cdn.com/v4/letter/z/f4b2a3/32.png) [@ZXY](https://gromacs.bioexcel.eu/u/ZXY)\
**Post date:** [August 5, 2026, 12:57am UTC](https://gromacs.bioexcel.eu/t/how-to-add-salt/13644/1 "2026-08-05T00:57:10Z")

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GROMACS version:  
GROMACS modification: Yes/No  
Here post your question  
I have a question for the community: for the same membrane+protein system in GROMACS with Martini, when using `insane` to add water and salt at `-salt 0.15`, I get NA: 2086, CL: 1106, W: 144943; whereas with `gmx genion -neutral -conc 0.15`, I get NA: 2965, CL: 1983, W: 146929. Which one is correct?

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**Author:** ![Marius](https://avatars.discourse-cdn.com/v4/letter/m/ed655f/32.png) [@Marius](https://gromacs.bioexcel.eu/u/Marius)\
**Post date:** [August 5, 2026, 1:15pm UTC](https://gromacs.bioexcel.eu/t/how-to-add-salt/13644/2 "2026-08-05T13:15:09Z")

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Hi there,

this is a delicate questions. Your systems seems to be highly charged (net charge approx. -1000e) you should be careful comparing actual ion concentrations with experiments.

Coming to your actual question: “Which one is correct?”

In my opinion, I would prefer the output of `insane`. `gmx genion` calculates ion concentrations based on the box volume, but a large fraction of the box volume is inaccessible to ions (membrane+protein). Your ion concentration in the bulk solvent would be therefore overestimated. `insane` should calculate it based on the amount of water in the system, excluding thereby effectively the volume taken up by the membrane and the protein. As a rule of thumb you can calculate ion concentrations with:

`n(NA+) = 0.15/55.55 * n(Water) * X - 0.5 * q(Solute)`

`n(CL-) = 0.15/55.55 * n(Water) * X + 0.5 * q(Solute)`

`0.15 mol/l` is the ion concentration and `55.55 mol/l` the molarity of water at 298K. `X` is 1 for an atomistic force field and 4 for the MARTINI force field (one water bead in MARTINI equals to 4 “real” water molecules). In your case this results in: 1076 CL- & 2056 NA+ . So pretty close to the numbers that `insane` has provided. It deviates because I assumed 145000 initial water beads that may change and `insane` deviates a little bit from this scheme (if I remember correctly). Please take a look into `insane.py` if you want to know it exactly.

All the best,  
Marius

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**Author:** ![ZXY](https://avatars.discourse-cdn.com/v4/letter/z/f4b2a3/32.png) [@ZXY](https://gromacs.bioexcel.eu/u/ZXY)\
**Post date:** [August 26, 2026, 1:01pm UTC](https://gromacs.bioexcel.eu/t/how-to-add-salt/13644/3 "2026-08-26T13:01:03Z")

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Thanks a lot, your advice has really helped me