# Question about modification of forcefield for non-standard residues

**URL:** https://gromacs.bioexcel.eu/t/question-about-modification-of-forcefield-for-non-standard-residues/11464
**Category:** User discussions
**Tags:** forcefield
**Created:** [February 5, 2025, 12:05pm UTC](https://gromacs.bioexcel.eu/t/question-about-modification-of-forcefield-for-non-standard-residues/11464 "2025-02-05T12:05:55Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![Lothar](https://avatars.discourse-cdn.com/v4/letter/l/b2d939/32.png) [@Lothar](https://gromacs.bioexcel.eu/u/Lothar)
#### Post date: [February 5, 2025, 12:05pm UTC](https://gromacs.bioexcel.eu/t/question-about-modification-of-forcefield-for-non-standard-residues/11464/1 "2025-02-05T12:05:55Z")

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GROMACS version: 2024.4  
GROMACS modification: No

Hi, I am totally new in molecular dynamics and Linux system. I wanted to use GROMACS for MD of protein that contains asymmetrically methylated arginine (ADMA). This residue is not in OPLS forcefield and I know I have to add this manually to files like residuetypes.dat, ffbonded.itp and ffnonbonded.itp. Also I could use other forcefields like charmm36. How to find or generate such parameters? I know they are some web tools like CHARMM-GUI, LibParGen or Vienna-PTM, but they are not used for ligands topologies? I did run my protein without modifications and it was fine, but after I try do do something with files I have all the time errors. I believe it is not an easy issue. Thank you for help in advance.