# ERROR 1 \[file lig.prm, line 4\]: Unknown bond\_atomtype CG2R51

**URL:** <https://gromacs.bioexcel.eu/t/error-1-file-lig-prm-line-4-unknown-bond-atomtype-cg2r51/5867>\
**Category:** User discussions\
**Tags:** mdp-parameters\
**Created:** [February 24, 2023, 3:27pm UTC](https://gromacs.bioexcel.eu/t/error-1-file-lig-prm-line-4-unknown-bond-atomtype-cg2r51/5867 "2023-02-24T15:27:54Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![jerry](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@jerry](https://gromacs.bioexcel.eu/u/jerry)\
**Post date:** [February 24, 2023, 3:27pm UTC](https://gromacs.bioexcel.eu/t/error-1-file-lig-prm-line-4-unknown-bond-atomtype-cg2r51/5867/1 "2023-02-24T15:27:54Z")

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error while adding ions  
used charmm36-july2022 lig.str created using CGenFF and cgenff\_charmm2gmx\_py3\_nx2.py used to create .prm and other files  
Unknown bond\_atomtype CG2R51  
what to do plz help

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**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [February 25, 2023, 6:50pm UTC](https://gromacs.bioexcel.eu/t/error-1-file-lig-prm-line-4-unknown-bond-atomtype-cg2r51/5867/2 "2023-02-25T18:50:37Z")

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CG2R51 is a core CGenFF atom type and comes from the CHARMM36 force field distribution. Are you trying to mix and match force fields? A CGenFF topology must be used with the CHARMM36 force field, nothing else.

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**Author:** ![jerry](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@jerry](https://gromacs.bioexcel.eu/u/jerry)\
**Post date:** [February 26, 2023, 6:01am UTC](https://gromacs.bioexcel.eu/t/error-1-file-lig-prm-line-4-unknown-bond-atomtype-cg2r51/5867/3 "2023-02-26T06:01:27Z")

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yes i used CGenFF topology file only. i am using gromacs 2020.1, is it the reason for the error?

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<div class="post-metadata">

**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [March 2, 2023, 2:53pm UTC](https://gromacs.bioexcel.eu/t/error-1-file-lig-prm-line-4-unknown-bond-atomtype-cg2r51/5867/4 "2023-03-02T14:53:15Z")

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I can’t tell you anything productive without seeing all your files (system topology, ligand topology and `.prm` file).