# Constrained dynamics of oxygen atoms of water molecules

**URL:** <https://gromacs.bioexcel.eu/t/constrained-dynamics-of-oxygen-atoms-of-water-molecules/10335>\
**Category:** User discussions\
**Created:** [October 14, 2024, 8:47am UTC](https://gromacs.bioexcel.eu/t/constrained-dynamics-of-oxygen-atoms-of-water-molecules/10335 "2024-10-14T08:47:33Z")\
**Posts on this page:** 1\
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**Author:** ![Karis](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@Karis](https://gromacs.bioexcel.eu/u/Karis)\
**Post date:** [October 19, 2024, 3:56am UTC](https://gromacs.bioexcel.eu/t/constrained-dynamics-of-oxygen-atoms-of-water-molecules/10335/2 "2024-10-19T03:56:52Z")

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Have you tried using position restraints? If you use gmx make\_ndx to create a group containing only the oxygen atoms, you can then use gmx genrestr to create position restraints for that group, it will place a harmonic force to keep the atoms in place.

I’m not sure about setting the oxygen mass super high, but I’d found from a previous post that [freeze groups don’t work for all simulation setups](https://gromacs.bioexcel.eu/t/freeze-group-issue/3003/3)

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