# How to calculate the number of interfacial water molecules?

**URL:** <https://gromacs.bioexcel.eu/t/how-to-calculate-the-number-of-interfacial-water-molecules/5280>\
**Category:** User discussions\
**Created:** [November 19, 2022, 6:23am UTC](https://gromacs.bioexcel.eu/t/how-to-calculate-the-number-of-interfacial-water-molecules/5280 "2022-11-19T06:23:38Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Sahil](https://avatars.discourse-cdn.com/v4/letter/s/f6c823/32.png) [@Sahil](https://gromacs.bioexcel.eu/u/Sahil)\
**Post date:** [November 19, 2022, 6:23am UTC](https://gromacs.bioexcel.eu/t/how-to-calculate-the-number-of-interfacial-water-molecules/5280/1 "2022-11-19T06:23:38Z")

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Hii all!

So I am studying protein DNA interactions. Can anyone guide me on how I can calculate the number of water molecules in the interface of protein and DNA?

Thank you!

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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:** [November 22, 2022, 2:59pm UTC](https://gromacs.bioexcel.eu/t/how-to-calculate-the-number-of-interfacial-water-molecules/5280/2 "2022-11-22T14:59:11Z")

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Hi,  
as far as I know there is not GMX tools doing this. But you could use gmx select ([https://manual.gromacs.org/current/onlinehelp/gmx-select.html](https://manual.gromacs.org/current/onlinehelp/gmx-select.html)) to identify those molecules and write them a dynamic index.

Here you find the syntax for gmx select  
[https://manual.gromacs.org/current/onlinehelp/selections.html?highlight=selection](https://manual.gromacs.org/current/onlinehelp/selections.html?highlight=selection)

I hope it helps  
Alessandra