# Fatal error while running gromacs. Atom N not found in building block 1DG while combining tdb &rtp

**URL:** <https://gromacs.bioexcel.eu/t/fatal-error-while-running-gromacs-atom-n-not-found-in-building-block-1dg-while-combining-tdb-rtp/7924>\
**Category:** User discussions\
**Tags:** pdb2gmx, forcefield, analysis-tools\
**Created:** [December 26, 2023, 10:01am UTC](https://gromacs.bioexcel.eu/t/fatal-error-while-running-gromacs-atom-n-not-found-in-building-block-1dg-while-combining-tdb-rtp/7924 "2023-12-26T10:01:35Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![YusufL](https://avatars.discourse-cdn.com/v4/letter/y/a587f6/32.png) [@YusufL](https://gromacs.bioexcel.eu/u/YusufL)\
**Post date:** [December 26, 2023, 10:01am UTC](https://gromacs.bioexcel.eu/t/fatal-error-while-running-gromacs-atom-n-not-found-in-building-block-1dg-while-combining-tdb-rtp/7924/1 "2023-12-26T10:01:35Z")

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GROMACS version:  
GROMACS modification: Yes/No  
Here post your question  
How can correct this. Fatal error while running gromacs. Atom N not found in building block 1DG while combining tdb &rtp

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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:** [January 3, 2024, 4:48pm UTC](https://gromacs.bioexcel.eu/t/fatal-error-while-running-gromacs-atom-n-not-found-in-building-block-1dg-while-combining-tdb-rtp/7924/2 "2024-01-03T16:48:46Z")

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Presumably you’re using an older version of the CHARMM36 port that had all the terminus types (proteins and nucleic acids) merged into one file. In that case, you have to manually choose chemically appropriate termini with `-ter`, otherwise everything is interpreted as protein.