# Modifying topology file

**URL:** <https://gromacs.bioexcel.eu/t/modifying-topology-file/686>\
**Category:** User discussions\
**Created:** [August 19, 2020, 2:50pm UTC](https://gromacs.bioexcel.eu/t/modifying-topology-file/686 "2020-08-19T14:50:24Z")\
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**Author:** ![Dr\_DBW](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/dr_dbw/32/42_2.png) [@Dr\_DBW](https://gromacs.bioexcel.eu/u/Dr_DBW)\
**Post date:** [August 20, 2020, 12:57am UTC](https://gromacs.bioexcel.eu/t/modifying-topology-file/686/4 "2020-08-20T00:57:11Z")

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How many waters are in the system? Are they continuous in the coordinate file? How many ions are needed to be added to reach the 2M concentration?

One of the developers or those with better knowledge of how the back end of the `genion` code works will probably need to provide a comment on what is happening here.

For reference, here is where you mentioned this problem previously - [Ion Concentration Force Fields](https://gromacs.bioexcel.eu/t/ion-concentration-force-fields/611/23)

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