# How to generate topology and .itp files for succinic acid and Zn²⁺ in GROMACS using OPLS-AA?

**URL:** <https://gromacs.bioexcel.eu/t/how-to-generate-topology-and-itp-files-for-succinic-acid-and-zn-in-gromacs-using-opls-aa/12579>\
**Category:** User discussions\
**Tags:** pdb2gmx, forcefield, mdrun, simulation-setup, topology\
**Created:** [August 28, 2025, 10:27am UTC](https://gromacs.bioexcel.eu/t/how-to-generate-topology-and-itp-files-for-succinic-acid-and-zn-in-gromacs-using-opls-aa/12579 "2025-08-28T10:27:59Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Samuel\_Reji\_Bits](https://avatars.discourse-cdn.com/v4/letter/s/c89c15/32.png) [@Samuel\_Reji\_Bits](https://gromacs.bioexcel.eu/u/Samuel_Reji_Bits)\
**Post date:** [August 28, 2025, 10:27am UTC](https://gromacs.bioexcel.eu/t/how-to-generate-topology-and-itp-files-for-succinic-acid-and-zn-in-gromacs-using-opls-aa/12579/1 "2025-08-28T10:27:59Z")

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I’m setting up a GROMACS simulation (10 nm box made with PACKMOL) containing water, Zn²⁺, and succinic acid using the OPLS-AA force field. Since succinic acid is not a biomolecule, pdb2gmx shows it as UNL, and external tools like LigParGen/ATB didn’t work properly. How can I generate consistent .top and .itp files for succinic acid and Zn²⁺ under OPLS-AA? If there is a better way to set up the system or an improved workflow, any insights would be really helpful.