Newest CHARMM36 port for GROMACS

We are pleased to announce an updated port of the CHARMM36 force field for GROMACS, available at

The July 2020 release corresponds to the annual update to the CHARMM36 force field. Notable revisions include:

  1. NBFIX terms for K+ and Ca2+ ions with carboxylates
  2. New nucleic acid model compounds
  3. CGenFF version 4.4, including amide base model compounds, boronic acid and bicyclic boron “ester” model compounds and associated parameters

Please let me know if you encounter any problems using the force field. Happy simulating!

Contact person (name and email address): Justin Lemkul (

How the work has been tested/reviewed: Forces and energies have been compared between CHARMM and GROMACS for a set of isolated molecules (gas phase) including all amino acids, nucleotides, a subset of lipids and other compounds

Please be advised of a new “bug fix” release of the CHARMM36 port. There were some missing NBFIXes related to Ca2+ ions in the July 2020 version that have now been correctly added. Please check your results carefully if you used the July 2020 version for simulations involving Ca2+ to see if the missing NBFIXes may have impacted your results.

The tarball is named/dated “feb2021” to reflect the date of its release, but forcefield.doc still refers to July 2020 as the version of the force field, because this does correspond to the release date of the updated CHARMM force field. We maintain access to the July 2020 version on the MacKerell lab website so you can compare the force field files and easily see the changes.

I apologize for this error and will be improving our conversion scripts so that no items like this get omitted in the future. Unfortunately, there’s a fair amount of manual work required to prepare CHARMM-formatted files for conversion, and sometimes copy-paste issues spring up, which I believe was the case here.