# Clarification on Iodide Parameters for CHARMM36 in GROMACS

**URL:** <https://gromacs.bioexcel.eu/t/clarification-on-iodide-parameters-for-charmm36-in-gromacs/12300>\
**Category:** User discussions\
**Tags:** forcefield, mdp-parameters\
**Created:** [June 30, 2025, 2:51pm UTC](https://gromacs.bioexcel.eu/t/clarification-on-iodide-parameters-for-charmm36-in-gromacs/12300 "2025-06-30T14:51:23Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![kumaran](https://avatars.discourse-cdn.com/v4/letter/k/96bed5/32.png) [@kumaran](https://gromacs.bioexcel.eu/u/kumaran)\
**Post date:** [June 30, 2025, 2:51pm UTC](https://gromacs.bioexcel.eu/t/clarification-on-iodide-parameters-for-charmm36-in-gromacs/12300/1 "2025-06-30T14:51:23Z")

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GROMACS version: 2022  
GROMACS modification: Yes/No  
Here post your question

Dear GROMACS users,  
I am working on a protein system that includes iodide (I⁻) and am using the CHARMM36 force field. To define non-bonded parameters for iodide, I referred to:  
Li et al., 2015 – “Systematic Parameterization of Monovalent Ions Employing the Nonbonded Model”  
The reported parameters are:  
• σ (sigma) = 2.567 Å  
• ε (epsilon) = 0.79269938 kcal/mol  
Sigma conversion (Å → nm, adjusted for CHARMM-style σ)  
Sigma = Rmin/2 \* 0.1 \* 2^(-1/6)\*2

Applying this, I got:  
σ=4.5738806×10−1nm  
Epsilon conversion (kcal/mol → kJ/mol)  
Using the conversion factor 1 kcal/mol = 4.184 kJ/mol, I did:  
ϵ=0.79269938×4.184=3.3166kJ/mol  
Since CHARMM36 in GROMACS, I’m unsure whether the epsilon should be:  
• 3.3166 kJ/mol (directly converted)  
or  
• 0.33166 kJ/mol, accounting for a potential ε/4 usage?  
Could someone confirm  
• What the correct ε value should be for iodide in CHARMM36 with GROMACS?  
• Should I define ε = 3.3166 or ε = 0.33166 in the [nonbond\_params] section?  
Any clarification or example of similar ion parameter incorporation would be highly appreciated.  
Thank you

Kumaran