# Position restraints causing mdrun to stall on step 0

**URL:** <https://gromacs.bioexcel.eu/t/position-restraints-causing-mdrun-to-stall-on-step-0/12909>\
**Category:** User discussions\
**Tags:** mdp-parameters, mdrun\
**Created:** [November 12, 2025, 6:22pm UTC](https://gromacs.bioexcel.eu/t/position-restraints-causing-mdrun-to-stall-on-step-0/12909 "2025-11-12T18:22:45Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![cglang](https://avatars.discourse-cdn.com/v4/letter/c/a183cd/32.png) [@cglang](https://gromacs.bioexcel.eu/u/cglang)\
**Post date:** [November 12, 2025, 6:22pm UTC](https://gromacs.bioexcel.eu/t/position-restraints-causing-mdrun-to-stall-on-step-0/12909/1 "2025-11-12T18:22:45Z")

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GROMACS version: 2024.4  
GROMACS modification: No

Hi all,

I’m having issues with Gromacs 2024.4. I’m running a membrane simulation with antimicrobial peptides. Our group typically uses a layer of frozen water beneath the membrane- this stops any stray peptides that cross the pbc from interfering with the inner membrane leaflet. I have no troubles equilibrating with a 10fs timestep, but once I try a 20fs timestep, mdrun immediately stalls at step 0 and will not progress any further.

I am confident it is an issue with the frozen water layer, which has strong position restraints on it. Any ideas on why the larger timestep could be causing my issue?

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**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [November 17, 2025, 9:18am UTC](https://gromacs.bioexcel.eu/t/position-restraints-causing-mdrun-to-stall-on-step-0/12909/2 "2025-11-17T09:18:56Z")

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The maximum time step in atomistic MD simulations is 2 or 2.5 fs (unless you use some tricks). Any simulation will crash when using a 10 fs time step.

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<div class="post-metadata">

**Author:** ![cglang](https://avatars.discourse-cdn.com/v4/letter/c/a183cd/32.png) [@cglang](https://gromacs.bioexcel.eu/u/cglang)\
**Post date:** [November 17, 2025, 12:28pm UTC](https://gromacs.bioexcel.eu/t/position-restraints-causing-mdrun-to-stall-on-step-0/12909/3 "2025-11-17T12:28:57Z")

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I should have mentioned- this is a coarse grain system!

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<div class="post-metadata">

**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [November 17, 2025, 4:54pm UTC](https://gromacs.bioexcel.eu/t/position-restraints-causing-mdrun-to-stall-on-step-0/12909/4 "2025-11-17T16:54:16Z")

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Then I have no clue. I suppose it’s Martini. Better ask there.