# CHARMM36 forcefield.itp vs local installation

**URL:** <https://gromacs.bioexcel.eu/t/charmm36-forcefield-itp-vs-local-installation/8834>\
**Category:** User discussions\
**Tags:** forcefield\
**Created:** [April 10, 2024, 2:05am UTC](https://gromacs.bioexcel.eu/t/charmm36-forcefield-itp-vs-local-installation/8834 "2024-04-10T02:05:28Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![tebg](https://avatars.discourse-cdn.com/v4/letter/t/f14d63/32.png) [@tebg](https://gromacs.bioexcel.eu/u/tebg)\
**Post date:** [April 10, 2024, 2:05am UTC](https://gromacs.bioexcel.eu/t/charmm36-forcefield-itp-vs-local-installation/8834/1 "2024-04-10T02:05:28Z")

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GROMACS version: 2023.3 (and 2024.1)  
GROMACS modification: Yes

When I’ve previously run simulation generated by CHARMM-GUI, I have always used the forcefield.itp file with the CHARMM36 GROMACS port. However, I also have CHARMM36 installed in my local GROMACS repository (with modifications to lipid.rtp to include decanoic acid). When I’m running simulations on membranes from memgen, I have been using the forcefield.itp file from older charmm-gui simulations, but I’m not sure if there is a better way to incorporate my native charmm36 force field (ex. adding the path to my topol.top file)- does anyone have advice or feedback on how to do this (or if I am ok to proceed with the forcefield.itp)? Thank you so much in advance!

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**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:** [April 10, 2024, 12:11pm UTC](https://gromacs.bioexcel.eu/t/charmm36-forcefield-itp-vs-local-installation/8834/2 "2024-04-10T12:11:38Z")

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If you want to be certain about it, then as you say, you can always add the absolute path to your `forcefield.itp`. If you have two versions, and one is in your current directory, then `gmx pdb2gmx` will usually ask you to choose, but once you have a pre-made topology, the best you can do is change the include path.

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**Author:** ![tebg](https://avatars.discourse-cdn.com/v4/letter/t/f14d63/32.png) [@tebg](https://gromacs.bioexcel.eu/u/tebg)\
**Post date:** [April 12, 2024, 5:19am UTC](https://gromacs.bioexcel.eu/t/charmm36-forcefield-itp-vs-local-installation/8834/3 "2024-04-12T05:19:32Z")

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Thank you so much!! I’ll play around with that now!