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Force fields produced by the Open Force Field Initiative
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Open Force Fields

Build Status DOI

This repository contains force fields released by the Open Force Field Initiative. These files are in SMIRKS Native Open Force Field (SMIRNOFF) format. Details about this new format are documented in our recent publication (doi:10.1021/acs.jctc.8b00640 or bioRxiv), 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 the openforcefield repository.

Each force field is currently available in two forms -- Both 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.

DOIs for each force field in this repository can be found in the following table:

Filename DOI FF line
openff-1.0.0.offxml DOI Parsley
openff_unconstrained-1.0.0.offxml DOI Parsley
openff-1.0.0-RC2.offxml DOI None
openff_unconstrained-1.0.0-RC2.offxml DOI None
openff-1.0.0-RC1.offxml None None
openff_unconstrained-1.0.0-RC1.offxml None None


conda install -c omnia openforcefields


Installing this package exposes an entry point that makes the openforcefield-forcefields/offxml directory easily accessible by other packages in the same python installation. If the Open Force Field toolkit is installed, it will automatically detect and use this entry point:

>>> from openforcefield.typing.engines.smirnoff import ForceField
>>> ff = ForceField('openff-1.0.0-RC1.offxml') 

Otherwise, the entry point can be accessed by querying the openforcefield.smirnoff_forcefield_directory entry point group.

>>> from pkg_resources import iter_entry_points
>>> for entry_point in iter_entry_points(group='openforcefield.smirnoff_forcefield_directory'):
...     print(entry_point.load()())

What it is

The provided OFFXML (force field) files are successive versions of a general-purpose small molecule force field, written in the SMIRNOFF format; this force field should cover all or almost all of drug-like chemical space, and illustrate some of the major functionality of the SMIRNOFF format as well as how it simplifies the specification of force field parameters in a compact and chemically sensible way.


Force fields in the openff-X.Y.Z line are descended from the SMIRNOFF99Frosst line of force fields. The first official release was made in September 2019 as a result of the Open Force Field Initiative's refitting efforts.

General versioning guidelines

Applicable in general to SMIRNOFF-format FFs produced by the Open Force Field Consortium

Force fields moving forward will be called name-X.Y.Z

  • X denotes some major change in functional form.
  • Y is the parameterization epoch / generation, or a minor change that can affect energy.
  • Z is a bugfix version -- e.g. something we've caught and corrected.



Contributors to the relevant .offxml files include: (Basically everyone in the consortium)

Special thanks go to John D. Chodera (MSKCC) for his initial implementation of openforcefield toolkits and the SMIRNOFF format.

Andrea Rizzi (MSKCC) and Jeff Wagner (OFF/UC Irvine) contributed to the Python and Conda infrastructure of this package.


Project based on the Computational Molecular Science Python Cookiecutter version 1.0.

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