# Add missing residue in rtp in gromos54 a7 force field

**URL:** <https://gromacs.bioexcel.eu/t/add-missing-residue-in-rtp-in-gromos54-a7-force-field/2257>\
**Category:** User discussions\
**Created:** [May 31, 2021, 11:19am UTC](https://gromacs.bioexcel.eu/t/add-missing-residue-in-rtp-in-gromos54-a7-force-field/2257 "2021-05-31T11:19:42Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![BIPASAkar](https://avatars.discourse-cdn.com/v4/letter/b/838e76/32.png) [@BIPASAkar](https://gromacs.bioexcel.eu/u/BIPASAkar)\
**Post date:** [May 31, 2021, 11:19am UTC](https://gromacs.bioexcel.eu/t/add-missing-residue-in-rtp-in-gromos54-a7-force-field/2257/1 "2021-05-31T11:19:42Z")

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GROMACS version: gromacs 2018.7  
GROMACS modification: Yes/No  
Here post your question : I am trying to perform protein ligand MD simulation, where I have used ATB for creating ligand topology. While “adding ion” step, it is showing CPos atom is not found, but it is present in ATP file of gromos 54-a7,not present in rtp file. So how to add this residue in rtp file ? please help me.

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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:** [June 11, 2021, 12:30pm UTC](https://gromacs.bioexcel.eu/t/add-missing-residue-in-rtp-in-gromos54-a7-force-field/2257/2 "2021-06-11T12:30:58Z")

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Hi,  
maybe you can find the answer to your problems by looking at the previuos post in this forum  
see for example  
[https://gromacs.bioexcel.eu/search?q=ATB](https://gromacs.bioexcel.eu/search?q=ATB)