# Could you explain to me what is comm-mode in GROMACS?

**URL:** <https://gromacs.bioexcel.eu/t/could-you-explain-to-me-what-is-comm-mode-in-gromacs/4577>\
**Category:** User discussions\
**Created:** [August 8, 2022, 3:54pm UTC](https://gromacs.bioexcel.eu/t/could-you-explain-to-me-what-is-comm-mode-in-gromacs/4577 "2022-08-08T15:54:47Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![JakubH](https://avatars.discourse-cdn.com/v4/letter/j/df788c/32.png) [@JakubH](https://gromacs.bioexcel.eu/u/JakubH)\
**Post date:** [August 8, 2022, 3:54pm UTC](https://gromacs.bioexcel.eu/t/could-you-explain-to-me-what-is-comm-mode-in-gromacs/4577/1 "2022-08-08T15:54:47Z")

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GROMACS version: 2020.2  
GROMACS modification: No  
I want to know something more about comm-mode. I see this option in the lipid bilayer-water simulations comm-mode: linear Why do we use it? I checked it on documentation but is not so much information about that. [https://manual.gromacs.org/documentation/2021/manual-2021.pdf](https://manual.gromacs.org/documentation/2021/manual-2021.pdf)  
Or maybe you can recommend some book or article about that?

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**Author:** ![ericirrgang](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/ericirrgang/32/438_2.png) [@ericirrgang](https://gromacs.bioexcel.eu/u/ericirrgang)\
**Post date:** [August 9, 2022, 9:08am UTC](https://gromacs.bioexcel.eu/t/could-you-explain-to-me-what-is-comm-mode-in-gromacs/4577/2 "2022-08-09T09:08:01Z")

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“comm” == _center-of-mass motion_

Conservation of momentum may be violated due to floating point rounding error or stochastic simulation techniques. It may be appropriate to make small corrections, or it may simply be convenient for visualization purposes.

But there are potential consequences for rigorous thermodynamic correctness. This is discussed in basic MD texts with respect to the _number of degrees of freedom_ in a system and the caveats of stochastic integrators. Subtleties of barostats and thermostats are ongoing areas of discussion in journals.

I hope that helps clarify the GROMACS documentation on the options available to users.

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<div class="post-metadata">

**Author:** ![JakubH](https://avatars.discourse-cdn.com/v4/letter/j/df788c/32.png) [@JakubH](https://gromacs.bioexcel.eu/u/JakubH)\
**Post date:** [August 10, 2022, 12:40pm UTC](https://gromacs.bioexcel.eu/t/could-you-explain-to-me-what-is-comm-mode-in-gromacs/4577/3 "2022-08-10T12:40:41Z")

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Thanks

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<div class="post-metadata">

**Author:** ![ericirrgang](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/ericirrgang/32/438_2.png) [@ericirrgang](https://gromacs.bioexcel.eu/u/ericirrgang)\
**Post date:** [August 10, 2022, 12:57pm UTC](https://gromacs.bioexcel.eu/t/could-you-explain-to-me-what-is-comm-mode-in-gromacs/4577/4 "2022-08-10T12:57:26Z")

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It might make more sense if you also note the `comm-grps` option. Clearly, as a user, you might choose different comm-removal mode depending on what you’re after, and there are different physical consequences in each case.