# ERROR 1 \[file ligand.itp, line 280\]: Duplicate atom index (30) in dih

**URL:** <https://gromacs.bioexcel.eu/t/error-1-file-ligand-itp-line-280-duplicate-atom-index-30-in-dih/8891>\
**Category:** User discussions\
**Created:** [April 16, 2024, 6:27am UTC](https://gromacs.bioexcel.eu/t/error-1-file-ligand-itp-line-280-duplicate-atom-index-30-in-dih/8891 "2024-04-16T06:27:02Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Zion\_john](https://avatars.discourse-cdn.com/v4/letter/z/b487fb/32.png) [@Zion\_john](https://gromacs.bioexcel.eu/u/Zion_john)\
**Post date:** [April 16, 2024, 6:27am UTC](https://gromacs.bioexcel.eu/t/error-1-file-ligand-itp-line-280-duplicate-atom-index-30-in-dih/8891/1 "2024-04-16T06:27:02Z")

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GROMACS version: 2024.1  
GROMACS modification: Yes/No  
Here post your question

ERROR 1 [file ligand.itp, line 280]:  
Duplicate atom index (30) in dihedrals

ERROR 2 [file ligand.itp, line 280]:  
Duplicate atom index (30) in dihedrals

ERROR 3 [file ligand.itp, line 280]:  
Duplicate atom index (30) in dihedrals

ERROR 4 [file ligand.itp, line 280]:  
No default Improper Dih. types

ligand.itp

 ![image](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/1/127af5b275f1964a79f7fc4ac872babdf9ebfe4b.png)

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<div class="post-metadata">

**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\
**Post date:** [April 16, 2024, 7:17am UTC](https://gromacs.bioexcel.eu/t/error-1-file-ligand-itp-line-280-duplicate-atom-index-30-in-dih/8891/2 "2024-04-16T07:17:04Z")

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As suggested by the error message, that is not a valid dihedral. You should have four different atoms in a dihedral angle. Did you get this itp file from somewhere else? If so, it would be good to report the error to them. If you made it yourself, I would recommend having a look at the reference manual, such as [Parameter files - GROMACS 2024.1 documentation](https://manual.gromacs.org/current/reference-manual/topologies/parameter-files.html#bonded-parameters) and [File formats - GROMACS 2024.1 documentation](https://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html#topology-file).