# Ion Concentration Force Fields

**URL:** <https://gromacs.bioexcel.eu/t/ion-concentration-force-fields/611>\
**Category:** User discussions\
**Created:** [August 1, 2020, 1:29am UTC](https://gromacs.bioexcel.eu/t/ion-concentration-force-fields/611 "2020-08-01T01:29:11Z")\
**Posts on this page:** 1\
**Showing post:** 23

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**Author:** ![L\_k](https://avatars.discourse-cdn.com/v4/letter/l/ebca7d/32.png) [@L\_k](https://gromacs.bioexcel.eu/u/L_k)\
**Post date:** [August 14, 2020, 2:52am UTC](https://gromacs.bioexcel.eu/t/ion-concentration-force-fields/611/23 "2020-08-14T02:52:01Z")

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When achieving a specified concentration, I usually replace water molecules with the ions. I notice that at extreme concentrations (above 1M) there reaches a point where there are no more water molecules that can be replaced for my current box size. Is the only solution to increase box size? Or is there some other method of reaching a certain concentration?

I am getting the ‘No more replaceable solvent!’ error.

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