# Is it allowed to use Charmm36 ff on protein and GAFF on ligand?

**URL:** <https://gromacs.bioexcel.eu/t/is-it-allowed-to-use-charmm36-ff-on-protein-and-gaff-on-ligand/10428>\
**Category:** User discussions\
**Tags:** forcefield\
**Created:** [October 28, 2024, 12:40pm UTC](https://gromacs.bioexcel.eu/t/is-it-allowed-to-use-charmm36-ff-on-protein-and-gaff-on-ligand/10428 "2024-10-28T12:40:46Z")\
**Posts on this page:** 1\
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**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:** [October 28, 2024, 3:17pm UTC](https://gromacs.bioexcel.eu/t/is-it-allowed-to-use-charmm36-ff-on-protein-and-gaff-on-ligand/10428/2 "2024-10-28T15:17:48Z")

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This is from a different context, but still valid: “If you are trying to mix two force fields, then you are asking for trouble.” ([Common errors when using GROMACS - GROMACS 2024.3 documentation](https://manual.gromacs.org/current/user-guide/run-time-errors.html#found-a-second-defaults-directive-file))

While it can be possible to make it work with a lot of effort, all your results will be questionable at best (probably outright wrong). You should make sure that the force fields you are using are compatible. Charmm36 and GAFF (Amber FF) are not compatible.

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