# How to determine the amount of solvent added to the simulation box?

**URL:** <https://gromacs.bioexcel.eu/t/how-to-determine-the-amount-of-solvent-added-to-the-simulation-box/3628>\
**Category:** User discussions\
**Tags:** simulation-setup\
**Created:** [February 9, 2022, 3:22am UTC](https://gromacs.bioexcel.eu/t/how-to-determine-the-amount-of-solvent-added-to-the-simulation-box/3628 "2022-02-09T03:22:16Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![ayyaxyz](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/ayyaxyz/32/1257_2.png) [@ayyaxyz](https://gromacs.bioexcel.eu/u/ayyaxyz)\
**Post date:** [February 9, 2022, 3:22am UTC](https://gromacs.bioexcel.eu/t/how-to-determine-the-amount-of-solvent-added-to-the-simulation-box/3628/1 "2022-02-09T03:22:16Z")

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GROMACS version: 2021.4  
GROMACS modification: No

I will do a simulation with various number of molecules A and B in TIP4P/2005 water as the solvent.  
Here are the variations:

- Variation 1 = 25 molecules of A and 5 molecules of B
- Variation 2 = 25 molecules of A and 10 molecules of B
- Variation 3 = 25 molecules of A and 15 molecules of B
- Variation 4 = 5 molecules of A and 25 molecules of B
- Variation 5 = 10 molecules of A and 25 molecules of B
- Variation 6 = 15 molecules of A and 25 molecules of B

Variations in the number of molecules are arranged using **Packmol** with a maximum random coordinate of 60 Angstroms along the x, y, and z axis (0. 0. 0. 60. 60. 60.).  
Then, I turned it into a simulation box with module

> gmx editconf -f initial.pdb -o box.gro -bt cubic -d 1.0

Then, I added the water molecule TIP4P/2005 using the module

> gmx solvate -cp box.gro -cs tip4p\_2005.gro -p topol.top -o solv.gro

I want the simulation system to only be affected by the difference in the number of molecules so that it’s like an experiment in a laboratory where the system will be more concentrated with a constant amount of solvent.

However, a problem arises, namely the amount of water solvent added is different for each variation. **The amount of water solvent increases with increasing molecular variation.**

**How to handle it?**  
**How to set the amount of TIP4P/2005 water solvent added to the simulation box for different number of molecules variation?**

Please help me,  
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:** [February 9, 2022, 8:47am UTC](https://gromacs.bioexcel.eu/t/how-to-determine-the-amount-of-solvent-added-to-the-simulation-box/3628/2 "2022-02-09T08:47:45Z")

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Hi,  
gmx solvate has an option to control the max number of solvent molecules ( `-maxsol`) . But not sure that solve your issue.  
Maybe an alternative is to look at A (or/and B) concentrations in water (if the experiments are performed at constant concentration of A and/or B in water,) in place of the number of water.  
\Alessandra

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**Author:** ![ayyaxyz](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/ayyaxyz/32/1257_2.png) [@ayyaxyz](https://gromacs.bioexcel.eu/u/ayyaxyz)\
**Post date:** [February 9, 2022, 10:49pm UTC](https://gromacs.bioexcel.eu/t/how-to-determine-the-amount-of-solvent-added-to-the-simulation-box/3628/3 "2022-02-09T22:49:54Z")

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I see, thank you for your enlightenment, Mrs. Alessandra.  
I will try it first.

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**Author:** ![Abhi1](https://avatars.discourse-cdn.com/v4/letter/a/e9c0ed/32.png) [@Abhi1](https://gromacs.bioexcel.eu/u/Abhi1)\
**Post date:** [October 21, 2024, 9:28am UTC](https://gromacs.bioexcel.eu/t/how-to-determine-the-amount-of-solvent-added-to-the-simulation-box/3628/4 "2024-10-21T09:28:22Z")

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Hi @ayyaxyz ,  
Could you please tell me how did you get/create the tip4p\_2005.gro file?

Thanks