# Cannot compile tpr file for CP2K QM/MM simulation with constraints = h-bonds

**URL:** <https://gromacs.bioexcel.eu/t/cannot-compile-tpr-file-for-cp2k-qm-mm-simulation-with-constraints-h-bonds/6203>\
**Category:** User discussions\
**Tags:** mdp-parameters\
**Created:** [April 17, 2023, 5:16am UTC](https://gromacs.bioexcel.eu/t/cannot-compile-tpr-file-for-cp2k-qm-mm-simulation-with-constraints-h-bonds/6203 "2023-04-17T05:16:32Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![AlbinP](https://avatars.discourse-cdn.com/v4/letter/a/c68b51/32.png) [@AlbinP](https://gromacs.bioexcel.eu/u/AlbinP)\
**Post date:** [April 17, 2023, 5:16am UTC](https://gromacs.bioexcel.eu/t/cannot-compile-tpr-file-for-cp2k-qm-mm-simulation-with-constraints-h-bonds/6203/1 "2023-04-17T05:16:32Z")

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GROMACS version: 2022.5 with CP2K 2023.1  
GROMACS modification: No

While running _grompp_ to create a tpr file for QM/MM simulation below warning regarding constraints is displayed.  
**WARNING 1 [file qmmm.mdp]:**  
Your QM subsystem has a lot of constrained bonds. They probably have been  
generated automatically. That could produce an artifacts in the  
simulation. Consider constraints = none in the mdp file.

When using constraints = none in the mdp file another warning is shown.  
**WARNING 1 [file qmmm.top, line 47]:**  
The bond in molecule-type Protein\_chain\_B between atoms 45 OG and 46 HG  
has an estimated oscillational period of 9.0e-03 ps, which is less than 5  
times the time step of 2.0e-03 ps. Maybe you forgot to change the constraints mdp option.

Can anyone help me on what is the issue using h-bond constraints and how can I proceed further.

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**Author:** ![Noob](https://avatars.discourse-cdn.com/v4/letter/n/5e9695/32.png) [@Noob](https://gromacs.bioexcel.eu/u/Noob)\
**Post date:** [May 12, 2024, 7:35am UTC](https://gromacs.bioexcel.eu/t/cannot-compile-tpr-file-for-cp2k-qm-mm-simulation-with-constraints-h-bonds/6203/2 "2024-05-12T07:35:22Z")

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Did you ever figure it out? I have the same issue.

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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:** [May 24, 2024, 2:32pm UTC](https://gromacs.bioexcel.eu/t/cannot-compile-tpr-file-for-cp2k-qm-mm-simulation-with-constraints-h-bonds/6203/3 "2024-05-24T14:32:46Z")

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Without h-bond constraints you need a time-step of about 0.5 fs. You will anyhow need this if the QM region includes hydrogens.