Release History

Releases follow the major.minor.micro scheme recommended by PEP440, where

  • major increments denote a change that may break API compatibility with previous major releases

  • minor increments add features but do not break API compatibility

  • micro increments represent bugfix releases or improvements in documentation

0.9.1 - Minor feature and bugfix release

New features

Behavior changed

  • PR #802: Fixes Issue #408. The 1-4 scaling factor for electrostatic interactions is now properly set by the value specified in the force field. Previously it fell back to a default value of 0.83333. The toolkit may now produce slightly different energies as a result of this change.

  • PR #839: The average WBO will now be returned when multiple conformers are provided to assign_fractional_bond_orders using use_conformers.

  • PR #816: Force field file paths are now loaded in a case-insensitive manner.


  • PR #849: Changes create_openmm_system so that it no longer uses the conformers on existing reference molecules (if present) to calculate Wiberg bond orders. Instead, new conformers are always generated during parameterization.

Improved documentation and warnings

  • PR #838: Corrects spacing of “forcefield” to “force field” throughout documentation. Fixes Issue #112.

  • PR #846: Corrects dead links throughout release history. Fixes Issue #835.

  • PR #847: Documentation now compiles with far fewer warnings, and in many cases more correctly. Additionally, ParameterAttribute documentation no longer appears incorrectly in classes where it is used. Fixes Issue #397.

0.9.0 - Namespace Migration

This release marks the transition from the old openforcefield branding over to its new identity as openff-toolkit. This change has been made to better represent the role of the toolkit, and highlight its place in the larger Open Force Field (OpenFF) ecosystem.

From version 0.9.0 onwards the toolkit will need to be imported as import openff.toolkit.XXX and from openff.toolkit import XXX.

API-breaking changes

  • PR #803: Migrates openforcefield imports to openff.toolkit.

0.8.4 - Minor feature and bugfix release

This release is intended to be functionally identical to 0.9.1. The only difference is that it uses the “openforcefield” namespace.

This release is a final patch for the 0.8.X series of releases of the toolkit, and also marks the last version of the toolkit which will be imported as import openforcefield.XXX / from openforcefield import XXX. From version 0.9.0 onwards the toolkit will be importable only as import openff.toolkit.XXX / from openff.toolkit import XXX.

Note This change will also be accompanied by a renaming of the package from openforcefield to openff-toolkit, so users need not worry about accidentally pulling in a version with changed imports. Users will have to explicitly choose to install the openff-toolkit package once released which will contain the breaking import changes.

0.8.3 - Major bugfix release

This release fixes a critical bug in van der Waals parameter assignment.

This release is also a final patch for the 0.8.X series of releases of the toolkit, and also marks the last version of the toolkit which will be imported as import openforcefield.XXX / from openforcefield import XXX. From version 0.9.0 onwards the toolkit will be importable only as import openff.toolkit.XXX / from openff.toolkit import XXX.

Note This change will also be accompanied by a renaming of the package from openforcefield to openff-toolkit, so users need not worry about accidentally pulling in a version with changed imports. Users will have to explicitly choose to install the openff-toolkit package once released which will contain the breaking import changes.


  • PR #808: Fixes Issue #807, which tracks a major bug in the interconversion between a vdW sigma and rmin_half parameter.

New features

  • PR #794: Adds a decorator @requires_package that denotes a function requires an optional dependency.

  • PR #805: Adds a deprecation warning for the up-coming release of the openff-toolkit package and its import breaking changes.

0.8.2 - Bugfix release

WARNING: This release was later found to contain a major bug, Issue #807, and produces incorrect energies.


  • PR #786: Fixes Issue #785 where RDKitToolkitWrapper would sometimes expect stereochemistry to be defined for non-stereogenic bonds when loading from SDF.

  • PR #786: Fixes an issue where using the Molecule copy constructor (newmol = Molecule(oldmol)) would result in the copy sharing the same .properties dict as the original (as in, changes to the .properties dict of the copy would be reflected in the original).

  • PR #789: Fixes a regression noted in Issue #788 where creating vdWHandler.vdWType or setting sigma or rmin_half using Quantities represented as strings resulted in an error.

0.8.1 - Bugfix and minor feature release

WARNING: This release was later found to contain a major bug, Issue #807, and produces incorrect energies.

API-breaking changes

  • PR #757: Renames test_forcefields/smirnoff99Frosst.offxml to test_forcefields/test_forcefield.offxml to avoid confusion with any of the ACTUAL released FFs in the smirnoff99Frosst line

  • PR #751: Removes the optional oetools=("oechem", "oequacpac", "oeiupac", "oeomega") keyword argument from OpenEyeToolkitWrapper.is_available, as there are no special behaviors that are accessed in the case of partially-licensed OpenEye backends. The new behavior of this method is the same as if the default value above is always provided.

Behavior Changed

New features


0.8.0 - Virtual Sites

This release implements the SMIRNOFF virtual site specification. The implementation enables support for models using off-site charges, including 4- and 5-point water models, in addition to lone pair modeling on various functional groups. The primary focus was on the ability to parameterize a system using virtual sites, and generating an OpenMM system with all virtual sites present and ready for evaluation. Support for formats other than OpenMM has not be implemented in this release, but may come with the appearance of the OpenFF system object. In addition to implementing the specification, the toolkit Molecule objects now allow the creation and manipulation of virtual sites.

Major Feature: Support for the SMIRNOFF VirtualSite tag

Virtual sites can be added to a System in two ways:

Virtual sites are the first parameters which directly depend on 3D conformation, where the position of the virtual sites are based on vectors defined on the atoms that were matched during parameterization. Because of this, a virtual site matching the triplet of atoms 1-2-3 will define a point that is different from a triplet matching 3-2-1. This is similar to defining “right-handed” and “left-handed” coordinate systems. This subtlety interplays with two major concepts in force field development:

  1. we sometimes want to define a single virtual site describing two points with the same parameters (distance, angle, etc.), such as 5-point water models

  2. we have a match that produces multiple orderings of the atoms (e.g. if wildcards are present in the SMARTS pattern), and we only want one to be applied.

Case 1) is very useful for parameter optimization, where a single SMARTS-based parameter can be used to optimize both points, such as the angle defining the virtual points for a 5-point water model. Case 2) is the typical scenario for the nitrogen lone pair in ammonia, where only one point needs to be specified. We discuss a few more illustrative examples below. Beyond these attributes, the virtual site specification allows a policy for specifying how to handle exclusions in the OpenMM force evaluator. The current default is to add pairwise energy exclusions in the OpenMM system between a virtual site and all tagged atoms matched in its SMARTS (exclusion_policy="parents", ). Currently defined are "none", "minimal", and "parents", where "minimal" specifies the single atom that the virtual site defines as the “origin”. For water, for example, "minimal" would mean just the oxygen, whereas "parents" would mean all three atoms.

In order to give consistent and intended behavior, the specification was modified from its draft form in following manner: The "name" and "match" attributes have been added to each virtual site parameter type. These changes allow for

  • specifying different virtual site types using the same atoms

  • allowing two virtual sites with the same type and same atoms but different physical parameters to be added simultaneously

  • allowing the ability to control whether the virtual site encodes one or multiple particles, based on the number of ways the matching atoms can be ordered.

The "name" attribute encodes whether the virtual site to be added should override an existing virtual site of the same type (e.g. hierarchy preference), or if this virtual site should be added in addition to the other existing virtual sites on the given atoms. This means that different virtual site types can share the same group of parent atoms and use the same name without overwriting each other (the default name is EP for all sites, which gives the expected hierarchical behavior used in other SMIRNOFF tags).

The "match" attribute accepts either "once" or "all_permutations", offering control for situations where a SMARTS pattern can possibly match the same group of atoms in different orders (either due to wildcards or local symmetry) and it is desired to either add just one or all of the possible virtual particles. The default value is "all_permutations", but for TrivalentLonePair it is always set to "once", regardless of what the file contains, since all orderings always place the particle in the exact same position.

The following cases exemplify our reasoning in implementing this behavior, and should draw caution to complex issues that may arise when designing virtual site parameters. Let us consider 4-, 5-, and 6-point water models:

  • A 4-point water model with a DivalentLonePair: This can be implemented by specifying match="once", outOfPlaneAngle="0*degree", and distance=-.15*angstrom". Since the SMIRKS pattern "[#1:1]-[#8X2:2]-[#2:3]" would match water twice and would create two particles in the exact same position if all_permutations was specified, we specify "once" to have only one particle generated. Although having two particles in the same position should not affect the physics if the proper exclusion policy is applied, it would effectively make the 4-point model just as expensive as 5-point models.

  • A 5-point water model with a DivalentLonePair: This can be implemented by using match="all_permutations" (unlike the 4-point model), outOfPlaneAngle="56.26*degree, and distance=0.7*angstrom, for example. Here the permutations will cause particles to be placed at ±56.26 degrees, and changing any of the physical quantities will affect both particles.

  • A 6-point water model with both DivalentLonePair sites above. Since these two parameters look identical, it is unclear whether they should both be applied or if one should override the other. The toolkit never compares the physical numbers to determine equality as this can lead to instability during e.g. parameter fitting. To get this to work, we specify name="EP1" for the first parameter, and name="EP2" for the second parameter. This instructs the parameter handler keep them separate, and therefore both are applied. (If both had the same name, then the typical SMIRNOFF hierarchy rules are used, and only the last matched parameter would be applied.)

  • Dinitrogen, N#N with a BondCharge virtual site. Since we want a BondCharge on both ends, we specify match="all_permutations".

  • Formaldehyde, H2C=O, with MonovalentLonePair virtual site(s) on the oxygen, with the aim of modeling both lone pairs. This one is subtle, since [#1:3]-[#6X3:2]=[#8X1:1] matches two unique groups of atoms (1-3-4 and 2-3-4). It is important to note in this situation that match="all_permutations" behaves exactly the same as match="once". Due to the anchoring hydrogens (1 and 2) being symmetric but opposite about the bond between 3 and 4, a single parameter does correctly place both lone pairs. A standing issue here is that the default exclusion policy (parents) will allow these two virtual sites to interact since they have different indexed atoms (parents), causing the energy to be different than the non-virtual site parameterization. In the future, the exclusion_policy="local" will account for this, and make virtual sites that share at least one “parent” atom not interact with each other. As a special note: when applying a MonovalentLonePair to a completely symmetric molecule, e.g. water, all_permutations can come into play, but this will apply two particles (one for each hydrogen).

Finally, the toolkit handles the organization of atoms and virtual sites in a specific manner. Virtual sites are expected to be added after all molecules in the topology are present. This is because the Open Force Field Toolkit organizes a topology by placing all atoms first, then all virtual sites last. This differs from the OpenMM Modeller object, for example, which interleaves the order of atoms and virtual sites in such a way that all particles of a molecule are contiguous. In addition, due to the fact that a virtual site may contain multiple particles coupled to single parameters, the toolkit makes a distinction between a virtual site, and a virtual particle. A virtual site may represent multiple virtual particles, so the total number of particles cannot be directly determined by simply summing the number of atoms and virtual sites in a molecule. This is taken into account, however, and the Molecule and Topology classes now implement particle iterators.

Minor Feature: Support for the 0.4 ChargeIncrementModel tag

To allow for more convenient fitting of ChargeIncrement parameters, it is now possible to specify one less charge_increment value than there are tagged atoms in a ChargeIncrement’s smirks. The missing charge_increment value will be calculated at parameterization-time to make the sum of the charge contributions from a ChargeIncrement parameter equal to zero. Since this change allows for force fields that are incompatible with the previous specification, this new style of ChargeIncrement must specify a ChargeIncrementModel section version of 0.4. All 0.3-compatible ChargeIncrement parameters are compatible with the 0.4 ChargeIncrementModel specification.

More details and examples of this change are available in The ChargeIncrementModel tag in the SMIRNOFF specification

New features


Behavior changed

  • PR #705: Changes the default values in the <Bonds> section of the SMIRNOFF spec to fractional_bondorder_method="AM1-Wiberg" and potential="(k/2)*(r-length)^2", which is backwards-compatible with and equivalent to potential="harmonic".

Examples added

  • PR #548: Adds a virtual site example notebook to run an OpenMM simulation with virtual sites, and compares positions and potential energy of TIP5P water between OpenFF and OpenMM force fields.

Tests added

  • PR #548: Adds test for

    • The virtual site parameter handler

    • TIP5P water dimer energy and positions

    • Adds tests to for virtual site/particle indexing/counting

0.7.2 - Bugfix and minor feature release

New features

Behavior changed

  • PR #684: Changes ToolkitRegistry to return an empty registry when initialized with no arguments, i.e. ToolkitRegistry() and makes the register_imported_toolkit_wrappers argument private.

  • PR #711: The setter for Topology.box_vectors now infers box vectors (a 3x3 matrix) when box lengths (a 3x1 array) are passed, assuming an orthogonal box.

  • PR #649: Makes SMARTS searches stereochemistry-specific (if stereo is specified in the SMARTS) for both OpenEye and RDKit backends. Also ensures molecule aromaticity is re-perceived according to the ForceField’s specified aromaticity model, which may overwrite user-specified aromaticity on the Molecule

  • PR #648: Removes the utils.structure module, which was deprecated in 0.2.0.

  • PR #670: Makes the Topology returned by create_openmm_system contain the partial charges and partial bond orders (if any) assigned during parameterization.

  • PR #675 changes the exception raised when no antechamber executable is found from IOError to AntechamberNotFoundError

  • PR #696 Adds an aromaticity_model keyword argument to the ForceField constructor, which defaults to DEFAULT_AROMATICITY_MODEL.


  • PR #715: Closes issue Issue #475 writing a “PDB” file using OE backend rearranges the order of the atoms by pushing the hydrogens to the bottom.

  • PR #649: Prevents 2020 OE toolkit from issuing a warning caused by doing stereo-specific smarts searches on certain structures.

  • PR #724: Closes issue Issue #502 Adding a utility function Topology.to_file() to write topology and positions to a “PDB” file using openmm backend for pdb file write.

Tests added

  • PR #694: Adds automated testing to code snippets in docs.

  • PR #715: Adds tests for pdb file writes using OE backend.

  • PR #724: Adds tests for the utility function Topology.to_file().

0.7.1 - OETK2020 Compatibility and Minor Update

This is the first of our patch releases on our new planned monthly release schedule.

Detailed release notes are below, but the major new features of this release are updates for compatibility with the new 2020 OpenEye Toolkits release, the get_available_force_fields function, and the disregarding of pyrimidal nitrogen stereochemistry in molecule isomorphism checks.

Behavior changed

  • PR #646: Checking for Molecule equality using the == operator now disregards all pyrimidal nitrogen stereochemistry by default. To re-enable, use Molecule.{is|are}_isomorphic with the strip_pyrimidal_n_atom_stereo=False keyword argument.

  • PR #646: Adds an optional toolkit_registry keyword argument to Molecule.are_isomorphic, which identifies the toolkit that should be used to search for pyrimidal nitrogens.


New features

0.7.0 - Charge Increment Model, Proper Torsion interpolation, and new Molecule methods

This is a relatively large release, motivated by the idea that changing existing functionality is bad so we shouldn’t do it too often, but when we do change things we should do it all at once.

Here’s a brief rundown of what changed, migration tips, and how to find more details in the full release notes below:

  • To provide more consistent partial charges for a given molecule, existing conformers are now disregarded by default by Molecule.assign_partial_charges. Instead, new conformers are generated for use in semiempirical calculations. Search for use_conformers.

  • Formal charges are now always returned as simtk.unit.Quantity objects, with units of elementary charge. To convert them to integers, use from simtk import unit and atom.formal_charge.value_in_unit(unit.elementary_charge) or mol.total_charge.value_in_unit(unit.elementary_charge). Search atom.formal_charge.

  • The OpenFF Toolkit now automatically reads and writes partial charges in SDF files. Search for atom.dprop.PartialCharges.

  • The OpenFF Toolkit now has different behavior for handling multi-molecule and multi-conformer SDF files. Search multi-conformer.

  • The OpenFF Toolkit now distinguishes between partial charges that are all-zero and partial charges that are unknown. Search partial_charges = None.

  • Topology.to_openmm now assigns unique atoms names by default. Search ensure_unique_atom_names.

  • Molecule equality checks are now done by graph comparison instead of SMILES comparison. Search Molecule.are_isomorphic.

  • The ChemicalEnvironment module was almost entirely removed, as it is an outdated duplicate of some Chemper functionality. Search ChemicalEnvironment.

  • TopologyMolecule.topology_particle_start_index has been removed from the TopologyMolecule API, since atoms and virtualsites are no longer contiguous in the Topology particle indexing system. Search topology_particle_start_index.

  • compute_wiberg_bond_orders has been renamed to assign_fractional_bond_orders.

There are also a number of new features, such as:

  • Support for ChargeIncrementModel sections in force fields.

  • Support for ProperTorsion k interpolation in force fields using fractional bond orders.

  • Support for AM1-Mulliken, Gasteiger, and other charge methods using the new assign_partial_charges methods.

  • Support for AM1-Wiberg bond order calculation using either the OpenEye or RDKit/AmberTools backends and the assign_fractional_bond_orders methods.

  • Initial (limited) interoperability with QCArchive, via Molecule.to_qcschema and from_qcschema.

  • A Molecule.visualize method.

  • Several additional Molecule methods, including state enumeration and mapped SMILES creation.

Major Feature: Support for the SMIRNOFF ChargeIncrementModel tag

The ChargeIncrementModel tag in the SMIRNOFF specification provides analagous functionality to AM1-BCC, except that instead of AM1-Mulliken charges, a number of different charge methods can be called, and instead of a fixed library of two-atom charge corrections, an arbitrary number of SMIRKS-based, N-atom charge corrections can be defined in the SMIRNOFF format.

The initial implementation of the SMIRNOFF ChargeIncrementModel tag accepts keywords for version, partial_charge_method, and number_of_conformers. partial_charge_method can be any string, and it is up to the ToolkitWrapper’s compute_partial_charges methods to understand what they mean. For geometry-independent partial_charge_method choices, number_of_conformers should be set to zero.

SMIRKS-based parameter application for ChargeIncrement parameters is different than other SMIRNOFF sections. The initial implementation of ChargeIncrementModelHandler follows these rules:

  • an atom can be subject to many ChargeIncrement parameters, which combine additively.

  • a ChargeIncrement that matches a set of atoms is overwritten only if another ChargeIncrement matches the same group of atoms, regardless of order. This overriding follows the normal SMIRNOFF hierarchy.

To give a concise example, what if a molecule A-B(-C)-D were being parametrized, and the force field defined ChargeIncrement SMIRKS in the following order?

  1. [A:1]-[B:2]

  2. [B:1]-[A:2]

  3. [A:1]-[B:2]-[C:3]

  4. [*:1]-[B:2](-[*:3])-[*:4]

  5. [D:1]-[B:2](-[*:3])-[*:4]

In the case above, the ChargeIncrement from parameters 1 and 4 would NOT be applied to the molecule, since another parameter matching the same set of atoms is specified further down in the parameter hierarchy (despite those subsequent matches being in a different order).

Ultimately, the ChargeIncrement contributions from parameters 2, 3, and 5 would be summed and applied.

It’s also important to identify a behavior that these rules were written to avoid: if not for the “regardless of order” clause in the second rule, parameters 4 and 5 could actually have been applied six and two times, respectively (due to symmetry in the SMIRKS and the use of wildcards). This situation could also arise as a result of molecular symmetry. For example, a methyl group could match the SMIRKS [C:1]([H:2])([H:3])([H:4]) six ways (with different orderings of the three hydrogen atoms), but the user would almost certainly not intend for the charge increments to be applied six times. The “regardless of order” clause was added specifically to address this.

In short, the first time a group of atoms becomes involved in a ChargeIncrement together, the System gains a new parameter “slot”. Only another ChargeIncrement which applies to the exact same group of atoms (in any order) can take over the “slot”, pushing the original ChargeIncrement out.

Major Feature: Support for ProperTorsion k value interpolation

Chaya Stern’s work showed that we may be able to produce higher-quality proper torsion parameters by taking into account the “partial bond order” of the torsion’s central bond. We now have the machinery to compute AM1-Wiberg partial bond orders for entire molecules using the assign_fractional_bond_orders methods of either OpenEyeToolkitWrapper or AmberToolsToolkitWrapper. The thought is that, if some simple electron population analysis shows that a certain aromatic bond’s order is 1.53, maybe rotations about that bond can be described well by interpolating 53% of the way between the single and double bond k values.

Full details of how to define a torsion-interpolating SMIRNOFF force fields are available in the ProperTorsions section of the SMIRNOFF specification.

Behavior changed

  • PR #508: In order to provide the same results for the same chemical species, regardless of input conformation, Molecule assign_partial_charges, compute_partial_charges_am1bcc, and assign_fractional_bond_orders methods now default to ignore input conformers and generate new conformer(s) of the molecule before running semiempirical calculations. Users can override this behavior by specifying the keyword argument use_conformers=molecule.conformers.

  • PR #281: Closes Issue #250 by adding support for partial charge I/O in SDF. The partial charges are stored as a property in the SDF molecule block under the tag <atom.dprop.PartialCharge>.

  • PR #281: If a Molecule’s partial_charges attribute is set to None (the default value), calling to_openeye will now produce a OE molecule with partial charges set to nan. This would previously produce an OE molecule with partial charges of 0.0, which was a loss of information, since it wouldn’t be clear whether the original OFFMol’s partial charges were REALLY all-zero as opposed to None. OpenEye toolkit wrapper methods such as from_smiles and from_file now produce OFFMols with partial_charges = None when appropriate (previously these would produce OFFMols with all-zero charges, for the same reasoning as above).

  • PR #281: Molecule to_rdkit now sets partial charges on the RDAtom’s PartialCharges property (this was previously set on the partial_charges property). If the Molecule’s partial_charges attribute is None, this property will not be defined on the RDAtoms.

  • PR #281: Enforce the behavior during SDF I/O that a SDF may contain multiple molecules, but that the OFF Toolkit does not assume that it contains multiple conformers of the same molecule. This is an important distinction, since otherwise there is ambiguity around whether properties of one entry in a SDF are shared among several molecule blocks or not, or how to resolve conflicts if properties are defined differently for several “conformers” of chemically-identical species (More info here). If the user requests the OFF Toolkit to write a multi-conformer Molecule to SDF, only the first conformer will be written. For more fine-grained control of writing properties, conformers, and partial charges, consider using Molecule.to_rdkit or Molecule.to_openeye and using the functionality offered by those packages.

  • PR #281: Due to different constraints placed on the data types allowed by external toolkits, we make our best effort to preserve Molecule properties when converting molecules to other packages, but users should be aware that no guarantee of data integrity is made. The only data format for keys and values in the property dict that we will try to support through a roundtrip to another toolkit’s Molecule object is string.

  • PR #574: Removed check that all partial charges are zero after assignment by quacpac when AM1BCC used for charge assignment. This check fails erroneously for cases in which the partial charge assignments are correctly all zero, such as for N#N. It is also an unnecessary check given that quacpac will reliably indicate when it has failed to assign charges.

  • PR #597: Energy-minimized sample systems with Parsley 1.1.0.

  • PR #558: The Topology particle indexing system now orders TopologyVirtualSites after all atoms.

  • PR #469: When running Topology.to_openmm, unique atom names are generated if the provided atom names are not unique (overriding any existing atom names). This uniqueness extends only to atoms in the same molecule. To disable this behavior, set the kwarg ensure_unique_atom_names=False.

  • PR #472: Molecule.__eq__ now uses the new Molecule.are_isomorphic to perform the similarity checking.

  • PR #472: The Topology.from_openmm and Topology.add_molecule methods now use the Molecule.are_isomorphic method to match molecules.

  • PR #551: Implemented the ParameterHandler.get_parameter function (would previously return None).

API-breaking changes

New features

Tests added


  • PR #558: Fixes a bug where TopologyVirtualSite.atoms would not correctly apply TopologyMolecule atom ordering on top of the reference molecule ordering, in cases where the same molecule appears multiple times, but in a different order, in the same Topology.

  • Issue #460: Creates unique atom names in Topology.to_openmm if the existing ones are not unique. The lack of unique atom names had been causing problems in workflows involving downstream tools that expect unique atom names.

  • Issue #448: We can now make molecules from mapped smiles using Molecule.from_mapped_smiles where the order will correspond to the indeing used in the smiles. Molecules can also be re-indexed at any time using the Molecule.remap.

  • Issue #462: We can now instance the Molecule from a QCArchive entry record instance or dictionary representation.

  • Issue #412: We can now instance the Molecule using Molecule.from_mapped_smiles. This resolves an issue caused by RDKit considering atom map indices to be a distinguishing feature of an atom, which led to erroneous definition of chirality (as otherwise symmetric substituents would be seen as different). We anticipate that this will reduce the number of times you need to type allow_undefined_stereo=True when processing molecules that do not actually contain stereochemistrty.

  • Issue #513: The Molecule.to_rdkit now re-sets the aromaticity model after sanitizing the molecule.

  • Issue #500: The Molecule.find_rotatable_bonds has been added which returns a list of rotatable Bond instances for the molecule.

  • Issue #491: We can now parse large molecules without hitting a match limit cap.

  • Issue #474: We can now convert molecules to InChI and InChIKey and from InChI.

  • Issue #523: The Molecule.to_file method can now correctly write to MOL files, in line with the supported file type list.

  • Issue #568: The Molecule.to_file can now correctly write multi-model PDB files when using the RDKit backend toolkit.

Examples added

0.6.0 - Library Charges

This release adds support for a new SMIRKS-based charge assignment method, Library Charges. The addition of more charge assignment methods opens the door for new types of experimentation, but also introduces several complex behaviors and failure modes. Accordingly, we have made changes to the charge assignment infrastructure to check for cases when partial charges do not sum to the formal charge of the molecule, or when no charge assignment method is able to generate charges for a molecule. More detailed explanation of the new errors that may be raised and keywords for overriding them are in the “Behavior Changed” section below.

With this release, we update test_forcefields/tip3p.offxml to be a working example of assigning LibraryCharges. However, we do not provide any force field files to assign protein residue LibraryCharges. If you are interested in translating an existing protein FF to SMIRNOFF format or developing a new one, please feel free to contact us on the Issue tracker or open a Pull Request.

New features

  • PR #433: Closes Issue #25 by adding initial support for the LibraryCharges tag in the SMIRNOFF specification using LibraryChargeHandler. For a molecule to have charges assigned using Library Charges, all of its atoms must be covered by at least one LibraryCharge. If an atom is covered by multiple LibraryCharge s, then the last LibraryCharge matched will be applied (per the hierarchy rules in the SMIRNOFF format).

    This functionality is thus able to apply per-residue charges similar to those in traditional protein force fields. At this time, there is no concept of “residues” or “fragments” during parametrization, so it is not possible to assign charges to some atoms in a molecule using LibraryCharge s, but calculate charges for other atoms in the same molecule using a different method. To assign charges to a protein, LibraryCharges SMARTS must be provided for the residues and protonation states in the molecule, as well as for any capping groups and post-translational modifications that are present.

    It is valid for LibraryCharge SMARTS to partially overlap one another. For example, a molecule consisting of atoms A-B-C connected by single bonds could be matched by a SMIRNOFF LibraryCharges section containing two LibraryCharge SMARTS: A-B and B-C. If listed in that order, the molecule would be assigned the A charge from the A-B LibraryCharge element and the B and C charges from the B-C element. In testing, these types of partial overlaps were found to frequently be sources of undesired behavior, so it is recommended that users define whole-molecule LibraryCharge SMARTS whenever possible.

  • PR #455: Addresses Issue #393 by adding ParameterHandler.attribute_is_cosmetic and ParameterType.attribute_is_cosmetic, which return True if the provided attribute name is defined for the queried object but does not correspond to an allowed value in the SMIRNOFF spec.

Behavior changed

  • PR #433: If a molecule can not be assigned charges by any charge-assignment method, an openforcefield.typing.engines.smirnoff.parameters.UnassignedMoleculeChargeException will be raised. Previously, creating a system without either ToolkitAM1BCCHandler or the charge_from_molecules keyword argument to ForceField.create_openmm_system would produce a system where the molecule has zero charge on all atoms. However, given that we will soon be adding more options for charge assignment, it is important that failures not be silent. Molecules with zero charge can still be produced by setting the Molecule.partial_charges array to be all zeroes, and including the molecule in the charge_from_molecules keyword argument to create_openmm_system.

  • PR #433: Due to risks introduced by permitting charge assignment using partially-overlapping LibraryCharge s, the toolkit will now raise a openforcefield.typing.engines.smirnoff.parameters.NonIntegralMoleculeChargeException if the sum of partial charges on a molecule are found to be more than 0.01 elementary charge units different than the molecule’s formal charge. This exception can be overridden by providing the allow_nonintegral_charges=True keyword argument to ForceField.create_openmm_system.

Tests added

  • PR #430: Added test for Wiberg Bond Order implemented in OpenEye Toolkits. Molecules taken from DOI:10.5281/zenodo.3405489 . Added by Sukanya Sasmal.

  • PR #569: Added round-trip tests for more serialization formats (dict, YAML, TOML, JSON, BSON, messagepack, pickle). Note that some are unsupported, but the tests raise the appropriate error.


  • PR #431: Fixes an issue where ToolkitWrapper objects would improperly search for functionality in the GLOBAL_TOOLKIT_REGISTRY, even though a specific ToolkitRegistry was requested for an operation.

  • PR #439: Fixes Issue #438, by replacing call to NetworkX Graph.node with call to Graph.nodes, per 2.4 migration guide.

Files modified

  • PR #433: Updates the previously-nonfunctional test_forcefields/tip3p.offxml to a functional state by updating it to the SMIRNOFF 0.3 specification, and specifying atomic charges using the LibraryCharges tag.

0.5.1 - Adding the parameter coverage example notebook

This release contains a new notebook example, check_parameter_coverage.ipynb, which loads sets of molecules, checks whether they are parameterizable, and generates reports of chemical motifs that are not. It also fixes several simple issues, improves warnings and docstring text, and removes unused files.

The parameter coverage example notebook goes hand-in-hand with the release candidate of our initial force field, openff-1.0.0-RC1.offxml , which will be temporarily available until the official force field release is made in October. Our goal in publishing this notebook alongside our first major refitting is to allow interested users to check whether there is parameter coverage for their molecules of interest. If the force field is unable to parameterize a molecule, this notebook will generate reports of the specific chemistry that is not covered. We understand that many organizations in our field have restrictions about sharing specific molecules, and the outputs from this notebook can easily be cropped to communicate unparameterizable chemistry without revealing the full structure.

The force field release candidate is in our new refit force field package, openforcefields. This package is now a part of the Open Force Field Toolkit conda recipe, along with the original smirnoff99Frosst line of force fields.

Once the openforcefields conda package is installed, you can load the release candidate using:

ff = ForceField('openff-1.0.0-RC1.offxml')

The release candidate will be removed when the official force field, openff-1.0.0.offxml, is released in early October.

Complete details about this release are below.

Example added

  • PR #419: Adds an example notebook check_parameter_coverage.ipynb which shows how to use the toolkit to check a molecule dataset for missing parameter coverage, and provides functionality to output tagged SMILES and 2D drawings of the unparameterizable chemistry.

New features

  • PR #419: Unassigned valence parameter exceptions now include a list of tuples of TopologyAtom which were unable to be parameterized (exception.unassigned_topology_atom_tuples) and the class of the ParameterHandler that raised the exception (exception.handler_class).

  • PR #425: Implements Trevor Gokey’s suggestion from Issue #411, which enables pickling of ForceFields and ParameterHandlers. Note that, while XML representations of ForceFields are stable and conform to the SMIRNOFF specification, the pickled ForceFields that this functionality enables are not guaranteed to be compatible with future toolkit versions.

Improved documentation and warnings

  • PR #425: Addresses Issue #410, by explicitly having toolkit warnings print Warning: at the beginning of each warning, and adding clearer language to the warning produced when the OpenEye Toolkits can not be loaded.

  • PR #425: Addresses Issue #421 by adding type/shape information to all Molecule partial charge and conformer docstrings.

  • PR #425: Addresses Issue #407 by providing a more extensive explanation of why we don’t use RDKit’s mol2 parser for molecule input.


Files removed

  • PR #425: Addresses Issue #424 by deleting the unused files openforcefield/typing/engines/smirnoff/ and openforcefield/tests/ was only used internally and tested unsupported functionality from before the 0.2.0 release.

0.5.0 - GBSA support and quality-of-life improvements

This release adds support for the GBSA tag in the SMIRNOFF specification. Currently, the HCT, OBC1, and OBC2 models (corresponding to AMBER keywords igb=1, 2, and 5, respectively) are supported, with the OBC2 implementation being the most flexible. Unfortunately, systems produced using these keywords are not yet transferable to other simulation packages via ParmEd, so users are restricted to using OpenMM to simulate systems with GBSA.

OFFXML files containing GBSA parameter definitions are available, and can be loaded in addition to existing parameter sets (for example, with the command ForceField('test_forcefields/smirnoff99Frosst.offxml', 'test_forcefields/GBSA_OBC1-1.0.offxml')). A manifest of new SMIRNOFF-format GBSA files is below.

Several other user-facing improvements have been added, including easier access to indexed attributes, which are now accessible as torsion.k1, torsion.k2, etc. (the previous access method torsion.k still works as well). More details of the new features and several bugfixes are listed below.

New features

  • PR #363: Implements GBSAHandler, which supports the GBSA tag in the SMIRNOFF specification. Currently, only GBSAHandlers with gb_model="OBC2" support setting non-default values for the surface_area_penalty term (default 5.4*calories/mole/angstroms**2), though users can zero the SA term for OBC1 and HCT models by setting sa_model="None". No model currently supports setting solvent_radius to any value other than 1.4*angstroms. Files containing experimental SMIRNOFF-format implementations of HCT, OBC1, and OBC2 are included with this release (see below). Additional details of these models, including literature references, are available on the SMIRNOFF specification page.


    The current release of ParmEd can not transfer GBSA models produced by the Open Force Field Toolkit to other simulation packages. These GBSA forces are currently only computable using OpenMM.

  • PR #363: When using Topology.to_openmm(), periodic box vectors are now transferred from the Open Force Field Toolkit Topology into the newly-created OpenMM Topology.

  • PR #377: Single indexed parameters in ParameterHandler and ParameterType can now be get/set through normal attribute syntax in addition to the list syntax.

  • PR #394: Include element and atom name in error output when there are missing valence parameters during molecule parameterization.


Files added

  • PR #363: Adds test_forcefields/GBSA_HCT-1.0.offxml, test_forcefields/GBSA_OBC1-1.0.offxml, and test_forcefields/GBSA_OBC2-1.0.offxml, which are experimental implementations of GBSA models. These are primarily used in validation tests against OpenMM’s models, and their version numbers will increment if bugfixes are necessary.

0.4.1 - Bugfix Release

This update fixes several toolkit bugs that have been reported by the community. Details of these bugfixes are provided below.

It also refactors how ParameterType and ParameterHandler store their attributes, by introducing ParameterAttribute and IndexedParameterAttribute. These new attribute-handling classes provide a consistent backend which should simplify manipulation of parameters and implementation of new handlers.

Bug fixes

  • PR #329: Fixed a bug where the two BondType parameter attributes k and length were treated as indexed attributes. (k and length values that correspond to specific bond orders will be indexed under k_bondorder1, k_bondorder2, etc when implemented in the future)

  • PR #329: Fixed a bug that allowed setting indexed attributes to single values instead of strictly lists.

  • PR #370: Fixed a bug in the API where BondHandler, ProperTorsionHandler , and ImproperTorsionHandler exposed non-functional indexed parameters.

  • PR #351: Fixes Issue #344, in which the main FrozenMolecule constructor and several other Molecule-construction functions ignored or did not expose the allow_undefined_stereo keyword argument.

  • PR #351: Fixes a bug where a molecule which previously generated a SMILES using one cheminformatics toolkit returns the same SMILES, even though a different toolkit (which would generate a different SMILES for the molecule) is explicitly called.

  • PR #354: Fixes the error message that is printed if an unexpected parameter attribute is found while loading data into a ForceField (now instructs users to specify allow_cosmetic_attributes instead of permit_cosmetic_attributes)

  • PR #364: Fixes Issue #362 by modifying OpenEyeToolkitWrapper.from_smiles and RDKitToolkitWrapper.from_smiles to make implicit hydrogens explicit before molecule creation. These functions also now raise an error if the optional keyword hydrogens_are_explicit=True but the SMILES are interpreted by the backend cheminformatic toolkit as having implicit hydrogens.

  • PR #371: Fixes error when reading early SMIRNOFF 0.1 spec files enclosed by a top-level SMIRFF tag.


The enclosing SMIRFF tag is present only in legacy files. Since developing a formal specification, the only acceptable top-level tag value in a SMIRNOFF data structure is SMIRNOFF.

Code enhancements

Force fields added

  • PR #368: Temporarily adds test_forcefields/smirnoff99frosst_experimental.offxml to address hierarchy problems, redundancies, SMIRKS pattern typos etc., as documented in issue #367. Will ultimately be propagated to an updated force field in the openforcefield/smirnoff99frosst repo.

  • PR #371: Adds test_forcefields/smirff99Frosst_reference_0_1_spec.offxml, a SMIRNOFF 0.1 spec file enclosed by the legacy SMIRFF tag. This file is used in backwards-compatibility testing.

0.4.0 - Performance optimizations and support for SMIRNOFF 0.3 specification

This update contains performance enhancements that significantly reduce the time to create OpenMM systems for topologies containing many molecules via ForceField.create_openmm_system.

This update also introduces the SMIRNOFF 0.3 specification. The spec update is the result of discussions about how to handle the evolution of data and parameter types as further functional forms are added to the SMIRNOFF spec.

We provide methods to convert SMIRNOFF 0.1 and 0.2 force fields written with the XML serialization (.offxml) to the SMIRNOFF 0.3 specification. These methods are called automatically when loading a serialized SMIRNOFF data representation written in the 0.1 or 0.2 specification. This functionality allows the toolkit to continue to read files containing SMIRNOFF 0.2 spec force fields, and also implements backwards-compatibility for SMIRNOFF 0.1 spec force fields.


The SMIRNOFF 0.1 spec did not contain fields for several energy-determining parameters that are exposed in later SMIRNOFF specs. Thus, when reading SMIRNOFF 0.1 spec data, the toolkit must make assumptions about the values that should be added for the newly-required fields. The values that are added include 1-2, 1-3 and 1-5 scaling factors, cutoffs, and long-range treatments for nonbonded interactions. Each assumption is printed as a warning during the conversion process. Please carefully review the warning messages to ensure that the conversion is providing your desired behavior.

SMIRNOFF 0.3 specification updates

  • The SMIRNOFF 0.3 spec introduces versioning for each individual parameter section, allowing asynchronous updates to the features of each parameter class. The top-level SMIRNOFF tag, containing information like aromaticity_model, Author, and Date, still has a version (currently 0.3). But, to allow for independent development of individual parameter types, each section (such as Bonds, Angles, etc) now has its own version as well (currently all 0.3).

  • All units are now stored in expressions with their corresponding values. For example, distances are now stored as 1.526*angstrom, instead of storing the unit separately in the section header.

  • The current allowed value of the potential field for ProperTorsions and ImproperTorsions tags is no longer charmm, but is rather k*(1+cos(periodicity*theta-phase)). It was pointed out to us that CHARMM-style torsions deviate from this formula when the periodicity of a torsion term is 0, and we do not intend to reproduce that behavior.

  • SMIRNOFF spec documentation has been updated with tables of keywords and their defaults for each parameter section and parameter type. These tables will track the allowed keywords and default behavior as updated versions of individual parameter sections are released.

Performance improvements and bugfixes

  • PR #329: Performance improvements when creating systems for topologies with many atoms.

  • PR #347: Fixes bug in charge assignment that occurs when charges are read from file, and reference and charge molecules have different atom orderings.

New features

  • PR #311: Several new experimental functions.

    • Adds convert_0_2_smirnoff_to_0_3, which takes a SMIRNOFF 0.2-spec data dict, and updates it to 0.3. This function is called automatically when creating a ForceField from a SMIRNOFF 0.2 spec OFFXML file.

    • Adds convert_0_1_smirnoff_to_0_2, which takes a SMIRNOFF 0.1-spec data dict, and updates it to 0.2. This function is called automatically when creating a ForceField from a SMIRNOFF 0.1 spec OFFXML file.

    • NOTE: The format of the “SMIRNOFF data dict” above is likely to change significantly in the future. Users that require a stable serialized ForceField object should use the output of ForceField.to_string('XML') instead.

    • Adds ParameterHandler and ParameterType add_cosmetic_attribute and delete_cosmetic_attribute functions. Once created, cosmetic attributes can be accessed and modified as attributes of the underlying object (eg. ParameterType.my_cosmetic_attrib = 'blue') These functions are experimental, and we are interested in feedback on how cosmetic attribute handling could be improved. (See Issue #338) Note that if a new cosmetic attribute is added to an object without using these functions, it will not be recognized by the toolkit and will not be written out during serialization.

    • Values for the top-level Author and Date tags are now kept during SMIRNOFF data I/O. If multiple data sources containing these fields are read, the values are concatenated using “AND” as a separator.

API-breaking changes

  • ForceField.to_string and ForceField.to_file have had the default value of their discard_cosmetic_attributes kwarg set to False.

  • ParameterHandler and ParameterType constructors now expect the version kwarg (per the SMIRNOFF spec change above) This requirement can be skipped by providing the kwarg skip_version_check=True

  • ParameterHandler and ParameterType functions no longer handle X_unit attributes in SMIRNOFF data (per the SMIRNOFF spec change above).

  • The scripts in utilities/convert_frosst are now deprecated. This functionality is important for provenance and will be migrated to the openforcefield/smirnoff99Frosst repository in the coming weeks.

  • ParameterType ._SMIRNOFF_ATTRIBS is now ParameterType ._REQUIRED_SPEC_ATTRIBS, to better parallel the structure of the ParameterHandler class.

  • ParameterType ._OPTIONAL_ATTRIBS is now ParameterType ._OPTIONAL_SPEC_ATTRIBS, to better parallel the structure of the ParameterHandler class.

  • Added class-level dictionaries ParameterHandler ._DEFAULT_SPEC_ATTRIBS and ParameterType ._DEFAULT_SPEC_ATTRIBS.

0.3.0 - API Improvements

Several improvements and changes to public API.

New features

  • PR #292: Implement Topology.to_openmm and remove ToolkitRegistry.toolkit_is_available

  • PR #322: Install directories for the lookup of OFFXML files through the entry point group openforcefield.smirnoff_forcefield_directory. The ForceField class doesn’t search in the data/forcefield/ folder anymore (now renamed data/test_forcefields/), but only in data/.

API-breaking Changes

  • PR #278: Standardize variable/method names

  • PR #291: Remove ForceField.load/to_smirnoff_data, add ForceField.to_file/string and ParameterHandler.add_parameters. Change behavior of ForceField.register_X_handler functions.


  • PR #327: Fix units in tip3p.offxml (note that this file is still not loadable by current toolkit)

  • PR #325: Fix solvent box for provided test system to resolve periodic clashes.

  • PR #325: Add informative message containing Hill formula when a molecule can’t be matched in Topology.from_openmm.

  • PR #325: Provide warning or error message as appropriate when a molecule is missing stereochemistry.

  • PR #316: Fix formatting issues in GBSA section of SMIRNOFF spec

  • PR #308: Cache molecule SMILES to improve system creation speed

  • PR #306: Allow single-atom molecules with all zero coordinates to be converted to OE/RDK mols

  • PR #313: Fix issue where constraints are applied twice to constrained bonds

0.2.2 - Bugfix release

This release modifies an example to show how to parameterize a solvated system, cleans up backend code, and makes several improvements to the README.


  • PR #279: Cleanup of unused code/warnings in main package __init__

  • PR #259: Update T4 Lysozyme + toluene example to show how to set up solvated systems

  • PR #256 and PR #274: Add functionality to ensure that links in READMEs resolve successfully

0.2.1 - Bugfix release

This release features various documentation fixes, minor bugfixes, and code cleanup.


  • PR #267: Add neglected <ToolkitAM1BCC> documentation to the SMIRNOFF 0.2 spec

  • PR #258: General cleanup and removal of unused/inaccessible code.

  • PR #244: Improvements and typo fixes for BRD4:inhibitor benchmark

0.2.0 - Initial RDKit support

This version of the toolkit introduces many new features on the way to a 1.0.0 release.

New features

  • Major overhaul, resulting in the creation of the SMIRNOFF 0.2 specification and its XML representation

  • Updated API and infrastructure for reference SMIRNOFF ForceField implementation

  • Implementation of modular ParameterHandler classes which process the topology to add all necessary forces to the system.

  • Implementation of modular ParameterIOHandler classes for reading/writing different serialized SMIRNOFF force field representations

  • Introduction of Molecule and Topology classes for representing molecules and biomolecular systems

  • New ToolkitWrapper interface to RDKit, OpenEye, and AmberTools toolkits, managed by ToolkitRegistry

  • API improvements to more closely follow PEP8 guidelines

  • Improved documentation and examples


This is an early preview release of the toolkit that matches the functionality described in the preprint describing the SMIRNOFF v0.1 force field format: [DOI].

New features

This release features additional documentation, code comments, and support for automated testing.


Treatment of improper torsions

A significant (though currently unused) problem in handling of improper torsions was corrected. Previously, non-planar impropers did not behave correctly, as six-fold impropers have two potential chiralities. To remedy this, SMIRNOFF impropers are now implemented as three-fold impropers with consistent chirality. However, current force fields in the SMIRNOFF format had no non-planar impropers, so this change is mainly aimed at future work.