open force field

An open and collaborative approach to better force fields

Force fields

Force fields released by the Open Force Field Initiative can be accessed in the GitHub repository.

The files available for download are in SMIRKS Native Open Force Field (SMIRNOFF) format. Details about this new format are documented in our recent publication (or preprint), and the most recent specification can be found in the Open Force Field Toolkit documentation. You can parameterize small molecules with SMIRNOFF using the ForceField class in the Open Force Field toolkit for simulations with OpenMM. The resulting system can also be converted to several other simulation formats using ParmEd. Usage examples can be found in our GitHub repository.




Parsley (OpenFF-1.y.z) is the codename for the first generation of optimized small molecule force fields from the Open Force Field Initiative. Parsley force fields represent a refit of the AMBER-lineage smirnoff99Frosst to large sets of quantum chemical data, and assign parameters using direct chemical perception, using the SMIRNOFF specification. Read more about the most important aspects of fitting and benchmarking procedures for Parsley in the blog post. See more information about naming and semantic versioning here.

All currently available versions are listed below. Each force field is currently available in two forms - with and without bond constraints to hydrogen. The default version of each force field is suitable for typical molecular dynamics simulations with constrained bonds to hydrogen. The unconstrained version of each force field should be used when single-point energies are a major concern (e.g. geometry optimizations) and when comparing the force field to QM data.


Release date: September 9, 2020

This bugfix release manually changes two bond force constants (b24 and b27) to resolve an issue seen in propyne substituents when using hydrogen mass repartitioning with a 4fs timestep. Full details are available at and in the reproduce_propyne_hmr.ipynb notebook in the release assets on GitHub.

GitHub DOI


Release date: June 1, 2020

This Parsley update contains the following improvements:

  • Dramatically improved QM fitting dataset increased parameter quality across the board, for almost all molecules tested;
  • Improved treatment of phosphonate groups;
  • Substantial improvements to torsions for molecules with single bonds between a divalent nitrogen and a trivalent nitrogen.

Read more details about this force field update in the release notes.


Datasets used for training and initial benchmarking of OpenFF-1.2.0 force field are listed below:

Dataset SMILES PDF Structures
[Fitting]( [SMI]( [PDF]( [Release tarball](
[Initial benchmark]( [SMI]( [PDF]( [Release tarball](
[Lim and Mobley benchmark](
_(in preparation, updates possible)_
[SMI]( [PDF]( [SDF](

The SMILES and PDF-format records are entirely derived from the structure files and may be updated without notice. If discrepancies are found, the structure files should be considered the authoritative reference.

GitHub DOI


Release date: April 14, 2020

This bugfix release contains the following changes: (1) Addition of monatomic ion LibraryCharges.

GitHub DOI


Release date: March 3, 2020

The first Parsley update contains results of valence parameter re-fitting, with small modifications in the input force field. The main highlights include:

  • Addition of two new bond terms and one angle term;
  • Modification of periodicity for N-N rotation;
  • Addition of some proper and improper torsions.

Read more details about the modified parameters in the release notes or in the blog post.

GitHub DOI


Release date: October 12, 2019

The first official small molecule force field release from the Open Force Field Initiative. Fitting and benchmarking details are described here.

GitHub DOI



A minimalist AMBER-compatible general small molecule force field with excellent coverage of druglike chemical space, using the SMIRNOFF direct chemical perception specification.

This force field achieves comparable accuracy to GAFF in physical properties, but with a decrease in complexity from over 6000 lines of parameters to only ~300 lines using the SMARTS-based direct chemical perception possible with SMIRNOFF. Read more about the latter and creation of smirnoff99Frosst in the publication or in the preprint.

smirnoff99Frosst was used as a starting point to generate the first optimized Open Force Field (Parsley) while building automated infrastructure for rapid parameterization of accurate small molecule force fields.

GitHub DOI