# Gmx make\_ndx cannot detect "chain" in GROMACS 2020

**URL:** <https://gromacs.bioexcel.eu/t/gmx-make-ndx-cannot-detect-chain-in-gromacs-2020/1484>\
**Category:** User discussions\
**Created:** [January 26, 2021, 9:31am UTC](https://gromacs.bioexcel.eu/t/gmx-make-ndx-cannot-detect-chain-in-gromacs-2020/1484 "2021-01-26T09:31:42Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![btam125](https://avatars.discourse-cdn.com/v4/letter/b/a88e57/32.png) [@btam125](https://gromacs.bioexcel.eu/u/btam125)\
**Post date:** [January 26, 2021, 9:31am UTC](https://gromacs.bioexcel.eu/t/gmx-make-ndx-cannot-detect-chain-in-gromacs-2020/1484/1 "2021-01-26T09:31:42Z")

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GROMACS version:2020 and 5.1.2  
GROMACS modification: No  
Here post your question

Dear all,

After trying to make an index file through .tpr, I found that the older version gromacs 5.1.2 can detect chains in the .tpr, while the gromacs 2020 cannot detect chain through the same command. I just wonder is this a bug?

The command that I put for both version:

gmx\_mpi make\_ndx -f topol.tpr

Best,

Ben

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**Author:** ![alevilla](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/alevilla/32/439_2.png) [@alevilla](https://gromacs.bioexcel.eu/u/alevilla)\
**Post date:** [January 26, 2021, 3:31pm UTC](https://gromacs.bioexcel.eu/t/gmx-make-ndx-cannot-detect-chain-in-gromacs-2020/1484/2 "2021-01-26T15:31:08Z")

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Hi  
gmx make\_ndx allows to split a group in chain (with the splitch), as soon the chains are properly defined in the topology file.  
Sorry, I do not recall what 5.1.2 was doing, you can have a look at the release notes if you want to know what is change and when [Release notes — GROMACS 2020.5 documentation](https://manual.gromacs.org/current/release-notes/index.html)

Best regards  
Alessandra

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**Author:** ![btam125](https://avatars.discourse-cdn.com/v4/letter/b/a88e57/32.png) [@btam125](https://gromacs.bioexcel.eu/u/btam125)\
**Post date:** [January 28, 2021, 6:30am UTC](https://gromacs.bioexcel.eu/t/gmx-make-ndx-cannot-detect-chain-in-gromacs-2020/1484/3 "2021-01-28T06:30:04Z")

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Hi,

I have read the release notes and I didn’t find anything regarding the gmx make\_ndx update since 2016. Hence I have no idea what happened.

Best,

Ben