# Topology file error after adding dihedral **URL:** \ **Category:** User discussions\ **Tags:** mdp-parameters\ **Created:** [May 28, 2024, 12:58pm UTC](https://gromacs.bioexcel.eu/t/topology-file-error-after-adding-dihedral/9241 "2024-05-28T12:58:46Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![TommasoChemEng](https://avatars.discourse-cdn.com/v4/letter/t/e36b37/32.png) [@TommasoChemEng](https://gromacs.bioexcel.eu/u/TommasoChemEng)\ **Post date:** [May 28, 2024, 12:58pm UTC](https://gromacs.bioexcel.eu/t/topology-file-error-after-adding-dihedral/9241/1 "2024-05-28T12:58:46Z")
GROMACS version: 2022.5 GROMACS modification: Plumed Dear Gromacs users and developers, I am trying to add a new residue using OPLS/AA force field. No problems until I added a custom dihedral angle in the aminoacids.rtp file, like this [dihedrals] C8 N1 C6 O1 dih\_1 In the ffbonded.itp files, I used this syntax to add the parameters: [dihedraltypes] #define dih\_1 3 … where I added the correct parameters for a RB type dihedral. When I generate the topology from a pdb using pdb2gmx, the topology file seems to be correct (no error messages about missing parameters for the ff) but this happens: [dihedrals] ; ai aj ak al funct 29 35 45 34 3 dih\_1 The indeces of the atoms and the function are correct, but it includes the definition of the dihedral. When I use grompp to generate the .tpr file, it complains because it finds more parameters than expected. I cannot find the mistake here. Have you some suggestions? I thank you in advance for your support. With my best regards, Tommaso ---
**Author:** ![milosz.wieczor](https://avatars.discourse-cdn.com/v4/letter/m/b19c9b/32.png) [@milosz.wieczor](https://gromacs.bioexcel.eu/u/milosz.wieczor)\ **Post date:** [May 29, 2024, 11:24am UTC](https://gromacs.bioexcel.eu/t/topology-file-error-after-adding-dihedral/9241/2 "2024-05-29T11:24:27Z")
The dihedral type (3 here) should not be part of the #defined variable - it will be inferred from the `.rtp` directive `[bondedtypes]` at the top of the file. So I guess instead of 6 values C0…C6 you’re introducing 7, and that’s what grompp complains about. ---
**Author:** ![TommasoChemEng](https://avatars.discourse-cdn.com/v4/letter/t/e36b37/32.png) [@TommasoChemEng](https://gromacs.bioexcel.eu/u/TommasoChemEng)\ **Post date:** [May 31, 2024, 12:50pm UTC](https://gromacs.bioexcel.eu/t/topology-file-error-after-adding-dihedral/9241/3 "2024-05-31T12:50:37Z")
Thanks for your reply, this solved my problem.