# Generation of .itp files for small molecules for OPLS and AMBER force fields in the GROMACS package

**URL:** <https://gromacs.bioexcel.eu/t/generation-of-itp-files-for-small-molecules-for-opls-and-amber-force-fields-in-the-gromacs-package/12347>\
**Category:** User discussions\
**Created:** [July 9, 2025, 6:28am UTC](https://gromacs.bioexcel.eu/t/generation-of-itp-files-for-small-molecules-for-opls-and-amber-force-fields-in-the-gromacs-package/12347 "2025-07-09T06:28:32Z")\
**Posts on this page:** 1\
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**Author:** ![lmullender](https://avatars.discourse-cdn.com/v4/letter/l/8dc957/32.png) [@lmullender](https://gromacs.bioexcel.eu/u/lmullender)\
**Post date:** [July 31, 2025, 12:33pm UTC](https://gromacs.bioexcel.eu/t/generation-of-itp-files-for-small-molecules-for-opls-and-amber-force-fields-in-the-gromacs-package/12347/4 "2025-07-31T12:33:16Z")

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Hi Devid,

Does that mean you tried it with other GROMACS versions and it worked? I don’t know what exactly your steps are, but like I said, the issue is not that OPLS and Amber force fields aren’t compatible with GROMACS, but that they not compatible with _each other_. That means, if you’re using e.g. LigParGen/OPLS to generate your ligand.itp file, you should also use the OPLS force field for the rest of your topology. GROMACS doesn’t care what the atom types are called, as long as they match. If you only tell it about the OPLS atom types like _opls\_XXX_, but then you add atoms that have an Amber-style atom type (e.g. _p5, oh_), gromacs won’t know what to do with that.

If you’re willing to share your .top and .itp files, maybe we could help you in more detail.

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