# Atom N not found in buiding block 1ACE while combining tdb and rtp. How to solve it?

**URL:** <https://gromacs.bioexcel.eu/t/atom-n-not-found-in-buiding-block-1ace-while-combining-tdb-and-rtp-how-to-solve-it/9316>\
**Category:** User discussions\
**Created:** [June 5, 2024, 3:26pm UTC](https://gromacs.bioexcel.eu/t/atom-n-not-found-in-buiding-block-1ace-while-combining-tdb-and-rtp-how-to-solve-it/9316 "2024-06-05T15:26:19Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Entissar](https://avatars.discourse-cdn.com/v4/letter/e/e9a140/32.png) [@Entissar](https://gromacs.bioexcel.eu/u/Entissar)\
**Post date:** [June 5, 2024, 3:26pm UTC](https://gromacs.bioexcel.eu/t/atom-n-not-found-in-buiding-block-1ace-while-combining-tdb-and-rtp-how-to-solve-it/9316/1 "2024-06-05T15:26:19Z")

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GROMACS version:2024.2  
GROMACS modification: Yes/No  
Dear all ,  
I’ve tried to perform an MD simulation with a protein-ligand complex. I am using the CHARMM27 FF GROMACS 2024.2 .

When running pdb2gmx it shows me the following error 👇

Fetal error:  
“atom N not found in building block 1ACE while combining tdb and rtp”

Can anyone help me to understand what may go wrong?  
Thanks in advance …

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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:** [June 5, 2024, 4:37pm UTC](https://gromacs.bioexcel.eu/t/atom-n-not-found-in-buiding-block-1ace-while-combining-tdb-and-rtp-how-to-solve-it/9316/2 "2024-06-05T16:37:45Z")

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For capped polypeptides, you need to select the appropriate terminal patching (None) with `gmx pdb2gmx -ter`, otherwise the program tries to build a free amino group onto the acetyl, which is chemically impossible.