# Obtaining CHARMM input files for GROMACS **URL:** \ **Category:** User discussions\ **Created:** [February 15, 2021, 5:08pm UTC](https://gromacs.bioexcel.eu/t/obtaining-charmm-input-files-for-gromacs/1600 "2021-02-15T17:08:44Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![megamence](https://avatars.discourse-cdn.com/v4/letter/m/5e9695/32.png) [@megamence](https://gromacs.bioexcel.eu/u/megamence)\ **Post date:** [February 15, 2021, 5:08pm UTC](https://gromacs.bioexcel.eu/t/obtaining-charmm-input-files-for-gromacs/1600/1 "2021-02-15T17:08:44Z")
GROMACS version: 2020 GROMACS modification: No Hello, I am trying to run a simple benzene simulation using the CHARMM forcefield. I managed to run a benzene simulation using GROMOS54A7\_atb using a .itp and structure file from the Automated Topology Builder. However, if I have a simple benzene pdb file which I made from Avogadro, how would I produce the .itp file I need to run the benzene simulation using GROMACS? Extrapolating from this, if I needed to run a simulation using some other force-field on GROMACS, for example, OPLS, how would I go about that? Apologies if this is a basic question, I am still trying to figure out the workings of GROMACS. ---
**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\ **Post date:** [February 16, 2021, 2:05pm UTC](https://gromacs.bioexcel.eu/t/obtaining-charmm-input-files-for-gromacs/1600/2 "2021-02-16T14:05:32Z")
CHARMM36 supports benzene (residue name BENZ in the `.rtp` file) so just set the residue name to BENZ in the input coordinate file and rename atoms as needed to match the `.rtp` entry. For other force fields, similar servers may exist (e.g. LigParGen for OPLS) but you can easily make an `.rtp` entry by simply looking at e.g. PHE and porting the charges and atom types to make benzene.