# Coordination number of water molecules in the solvation shell

**URL:** <https://gromacs.bioexcel.eu/t/coordination-number-of-water-molecules-in-the-solvation-shell/4500>\
**Category:** User discussions\
**Tags:** mdrun, analysis-tools\
**Created:** [July 23, 2022, 1:57pm UTC](https://gromacs.bioexcel.eu/t/coordination-number-of-water-molecules-in-the-solvation-shell/4500 "2022-07-23T13:57:12Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Kal](https://avatars.discourse-cdn.com/v4/letter/k/9de053/32.png) [@Kal](https://gromacs.bioexcel.eu/u/Kal)\
**Post date:** [July 23, 2022, 1:57pm UTC](https://gromacs.bioexcel.eu/t/coordination-number-of-water-molecules-in-the-solvation-shell/4500/1 "2022-07-23T13:57:12Z")

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GROMACS version: 2021.4  
GROMACS modification: No  
Here post your question

Hi All,  
I am doing MD simulations for halides in water. I need to calculate the average coordination number of water molecules in the solvation shell of the ions. How can I do that using GROMACS?

Cheers, Kal

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**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:** [July 25, 2022, 1:34pm UTC](https://gromacs.bioexcel.eu/t/coordination-number-of-water-molecules-in-the-solvation-shell/4500/2 "2022-07-25T13:34:58Z")

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Hi,  
to evaluate the coordination number, you can use the tool gmx rdf ( [gmx rdf — GROMACS 2022.2 documentation](https://manual.gromacs.org/current/onlinehelp/gmx-rdf.html)) with the option -cn  
\Alessandra