# Question on constraints (in \*.top and \*.mdp files)

**URL:** <https://gromacs.bioexcel.eu/t/question-on-constraints-in-top-and-mdp-files/9777>\
**Category:** User discussions\
**Created:** [July 30, 2024, 8:06pm UTC](https://gromacs.bioexcel.eu/t/question-on-constraints-in-top-and-mdp-files/9777 "2024-07-30T20:06:14Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![weil](https://avatars.discourse-cdn.com/v4/letter/w/ac91a4/32.png) [@weil](https://gromacs.bioexcel.eu/u/weil)\
**Post date:** [July 30, 2024, 8:06pm UTC](https://gromacs.bioexcel.eu/t/question-on-constraints-in-top-and-mdp-files/9777/1 "2024-07-30T20:06:14Z")

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GROMACS version: 2024.2  
GROMACS modification: Yes/No

I’m new to GROMACS and a little confused about the constraints in the topology and mdp files.

In the topology file, we can use [constraints] to set up, e.g., the bond length constraints. In the mdp file, we also have a constraints keyword, which can be none/h-bonds/all-bonds/h-angles/all-angles.

If constraints = all-bonds, does it mean it will convert all bonds to constraints, or just the bonds under [constraints] in the topology file? I am assuming the latter, but I would like to confirm. Thanks.

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**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\
**Post date:** [August 12, 2024, 8:10am UTC](https://gromacs.bioexcel.eu/t/question-on-constraints-in-top-and-mdp-files/9777/2 "2024-08-12T08:10:19Z")

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If using `constraints = all-bonds` **all** bonds will be converted to constraints. In most cases `constraints = h-bonds` is enough to enable a 2 fs time step.

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**Author:** ![weil](https://avatars.discourse-cdn.com/v4/letter/w/ac91a4/32.png) [@weil](https://gromacs.bioexcel.eu/u/weil)\
**Post date:** [September 6, 2024, 1:14pm UTC](https://gromacs.bioexcel.eu/t/question-on-constraints-in-top-and-mdp-files/9777/3 "2024-09-06T13:14:32Z")

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Thanks for confirming. What happens if I use constraints = none in the mdp file but set the constraints under [constraints] in the topology file? I assume the bonds under [constraints] will still be constrained?

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**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\
**Post date:** [September 6, 2024, 1:59pm UTC](https://gromacs.bioexcel.eu/t/question-on-constraints-in-top-and-mdp-files/9777/4 "2024-09-06T13:59:08Z")

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Yes, that is correct.

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**Author:** ![weil](https://avatars.discourse-cdn.com/v4/letter/w/ac91a4/32.png) [@weil](https://gromacs.bioexcel.eu/u/weil)\
**Post date:** [September 6, 2024, 3:56pm UTC](https://gromacs.bioexcel.eu/t/question-on-constraints-in-top-and-mdp-files/9777/5 "2024-09-06T15:56:54Z")

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Thanks for confirming.