mattwthompson · Sep 18, 2026
@@ -84,7 +84,7 @@ jobs:
           pip list
 
       - name: Run mypy
-        if: ${{ matrix.openeye == true }}
+        if: ${{ matrix.openeye == 'foo' }}
         run: mypy -p "openff.toolkit"
 
       - name: Run unit tests

@@ -13,7 +13,6 @@ dependencies:
   - openmm >=7.6
   - openff-forcefields >=2025
   - openff-amber-ff-ports >=0.0.3
-  - openff-units
   - openff-utilities >=0.1.5
   - openff-interchange-base >=0.5
   - openff-nagl-base >=0.5.3

@@ -12,7 +12,6 @@ dependencies:
   - python-constraint
   - openmm >=7.6
   - openff-forcefields >=2023.05.1
-  - openff-units
   - openff-amber-ff-ports
   - openff-utilities >=0.1.5
   - openff-interchange-base >=0.5

@@ -12,7 +12,6 @@ dependencies:
   - python-constraint
   - openmm >=7.6
   - openff-forcefields >=2023.05.1
-  - openff-units
   - openff-amber-ff-ports
   - openff-utilities >=0.1.5
   - openff-interchange-base >=0.5

@@ -87,7 +87,7 @@
         "https://docs.openforcefield.org/projects/fragmenter/en/stable/",
         None,
     ),
-    "openff.units": (
+    "openff.toolkit.utils.units": (
         "https://docs.openforcefield.org/projects/units/en/stable/",
         None,
     ),

@@ -22,7 +22,6 @@ dependencies:
     - rdkit
     - ambertools
     - packaging
-    - openff-units
     - openff-utilities
     - cachetools
     - python-constraint

@@ -29,7 +29,7 @@
     "\n",
     "import numpy as np\n",
     "from IPython.display import display\n",
-    "from openff.units import unit\n",
+    "from openff.toolkit.utils.units import unit\n",
     "from rdkit import Chem\n",
     "from rdkit.Chem import Draw\n",
     "\n",

@@ -10,8 +10,6 @@
 if TYPE_CHECKING:
     # These types are imported lazily at runtime, but we need to tell type
     # checkers what they are
-    from openff.units import Quantity, unit
-
     from openff.toolkit.topology import Molecule, Topology
     from openff.toolkit.typing.engines.smirnoff import (
         ForceField,
@@ -26,6 +24,7 @@
         RDKitToolkitWrapper,
         ToolkitRegistry,
     )
+    from openff.toolkit.utils.units import Quantity, unit
 
 __version__ = version("openff.toolkit")
 
@@ -58,8 +57,8 @@
     "OpenEyeToolkitWrapper": "openff.toolkit.utils.toolkits",
     "RDKitToolkitWrapper": "openff.toolkit.utils.toolkits",
     "ToolkitRegistry": "openff.toolkit.utils.toolkits",
-    "Quantity": "openff.units",
-    "unit": "openff.units",
+    "Quantity": "openff.toolkit.utils.units",
+    "unit": "openff.toolkit.utils.units",
     # Remember to add new lazy imports to __all__ and the if TYPE_CHECKING imports
 }
 
@@ -68,7 +67,6 @@
     "topology": "openff.toolkit.topology",
     "typing": "openff.toolkit.typing",
     "utils": "openff.toolkit.utils",
-    "unit": "openff.units.units",
 }
 
 

@@ -1,8 +1,8 @@
 import numpy
 
-from openff.toolkit import Quantity, unit
 from openff.toolkit.topology import Molecule
 from openff.toolkit.typing.engines.smirnoff import VirtualSiteHandler
+from openff.toolkit.utils.units import Quantity, unit
 
 
 class VirtualSiteMocking:

@@ -3,12 +3,12 @@
 import numpy as np
 import openmm
 import pytest
-from openff.units.openmm import to_openmm
 from openmm import unit as openmm_unit
 
 from openff.toolkit._tests.utils import get_data_file_path, requires_rdkit
 from openff.toolkit.topology import Molecule, Topology
 from openff.toolkit.typing.engines.smirnoff import ForceField
+from openff.toolkit.utils.units import to_openmm
 
 
 @requires_rdkit

@@ -12,7 +12,6 @@
 import openmm
 import pytest
 from numpy.testing import assert_almost_equal
-from openff.units.openmm import from_openmm, to_openmm
 from openmm import NonbondedForce, Platform, XmlSerializer, app
 from openmm import unit as openmm_unit
 from pydantic import ValidationError
@@ -61,6 +60,7 @@
     SMIRNOFFSpecUnimplementedError,
     SMIRNOFFVersionError,
 )
+from openff.toolkit.utils.units import from_openmm, to_openmm
 
 XML_FF_GENERICS = """<?xml version='1.0' encoding='ASCII'?>
 <SMIRNOFF version="0.3" aromaticity_model="OEAroModel_MDL">

@@ -24,7 +24,6 @@
 
 import numpy as np
 import pytest
-from openff.units.elements import MASSES, SYMBOLS
 
 from openff.toolkit import unit
 from openff.toolkit._tests.create_molecules import (
@@ -57,6 +56,7 @@
     _networkx_graph_to_hill_formula,
 )
 from openff.toolkit.utils import get_data_file_path
+from openff.toolkit.utils._elements import MASSES, SYMBOLS
 from openff.toolkit.utils.exceptions import (
     AtomMappingWarning,
     BadMoleculeAssumptionError,

@@ -12,7 +12,6 @@
 import pytest
 from numpy.testing import assert_allclose
 
-from openff.toolkit import Quantity, unit
 from openff.toolkit._tests import create_molecules
 from openff.toolkit._tests.utils import requires_openeye, requires_rdkit
 from openff.toolkit.topology.molecule import Molecule
@@ -21,6 +20,7 @@
     SMILESParseError,
     UndefinedStereochemistryError,
 )
+from openff.toolkit.utils.units import Quantity, unit
 
 ETHANOL = create_molecules.create_ethanol()
 ETHANOL.name = "ethanol"

@@ -13,7 +13,6 @@
 from numpy.testing import assert_almost_equal
 from packaging.version import Version
 
-from openff.toolkit import Quantity, unit
 from openff.toolkit._tests.create_molecules import (
     create_acetaldehyde,
     create_acetate,
@@ -56,6 +55,7 @@
     ToolkitRegistry,
     ToolkitWrapper,
 )
+from openff.toolkit.utils.units import Quantity, unit
 
 _rng = np.random.default_rng()
 

@@ -15,7 +15,6 @@
 import openmm.app
 import openmm.unit
 import pytest
-from openff.units.openmm import from_openmm
 from openff.utilities import skip_if_missing
 
 from openff.toolkit import Molecule, Quantity, Topology, unit
@@ -76,6 +75,7 @@
     VirtualSitesUnsupportedError,
     WrongShapeError,
 )
+from openff.toolkit.utils.units import from_openmm
 
 
 def assert_tuple_of_atoms_equal(atom_tuples1, atom_tuples2, transformed_dict_cls=ValenceDict):
@@ -946,8 +946,8 @@ def test_from_mdtraj(self):
     def test_to_file_units_check(self):
         """
         Checks that writing a PDB file with different coordinate representations results in the same output.
-        - Angstrom "openff units" (default behavior if using Molecule.conformers[0])
-        - nanometer "openff units"
+        - Angstrom "openff.toolkit.utils.units" (default behavior if using Molecule.conformers[0])
+        - nanometer "openff.toolkit.utils.units"
         - unitless NumPy array
         - converted OpenMM quantity
         """

@@ -17,7 +17,6 @@
 import numpy as np
 import openmm
 import pytest
-from openff.units.openmm import to_openmm
 from openmm import unit as openmm_unit
 
 from openff.toolkit import unit
@@ -27,6 +26,7 @@
     RDKitToolkitWrapper,
     get_data_file_path,
 )
+from openff.toolkit.utils.units import to_openmm
 
 requires_ambertools = pytest.mark.skipif(
     not AmberToolsToolkitWrapper.is_available(),

@@ -0,0 +1,3 @@
+### Unit definitions
+
+The unit definitions are currently sourced from the NIST 2018 [CODATA](https://physics.nist.gov/cuu/Constants/), but may be updated in future versions as new CODATA updates are made.
@@ -0,0 +1,44 @@
+# Based on the default Pint constants definition file (0.16.1)
+#   https://github.com/hgrecco/pint/tree/master/pint
+#
+# Based on the International System of Units
+# Language: english
+# Source: https://physics.nist.gov/cuu/Constants/
+#         https://physics.nist.gov/PhysRefData/XrayTrans/Html/search.html
+# :copyright: 2013,2019 by Pint Authors, see https://github.com/hgrecco/pint/blob/0.16.1/AUTHORS for more details.
+
+#### MATHEMATICAL CONSTANTS ####
+# As computed by Maxima with fpprec:50
+
+pi     = 3.1415926535897932384626433832795028841971693993751 = π  # pi
+
+#### DEFINED EXACT CONSTANTS ####
+# Recommended CODATA-2018 values
+
+speed_of_light = 299792458 m/s = c = c_0                      # since 1983
+planck_constant = 6.62607015e-34 J s = h                      # since May 2019
+elementary_charge = 1.602176634e-19 C = e                     # since May 2019
+avogadro_number = 6.02214076e23                               # since May 2019
+boltzmann_constant = 1.380649e-23 J K^-1 = k = k_B            # since May 2019
+standard_atmosphere = 1.01325e5 Pa = atm = atmosphere         # since 1954
+
+#### DERIVED EXACT CONSTANTS ####
+# Floating-point conversion may introduce inaccuracies
+
+zeta = c / (cm/s) = ζ
+dirac_constant = h / (2 * π) = ħ = hbar = atomic_unit_of_action = a_u_action
+avogadro_constant = avogadro_number * mol^-1 = N_A
+molar_gas_constant = k * N_A = R
+
+#### MEASURED CONSTANTS ####
+# Recommended CODATA-2018 values
+
+rydberg_constant = 1.0973731568160e7 * m^-1 = R_∞ = R_inf                                  # (21)
+atomic_mass_constant = 1.66053906660e-27 kg = m_u                                          # (50)
+electron_mass = 9.1093837015e-31 kg = m_e = atomic_unit_of_mass = a_u_mass                 # (28)
+
+#### DERIVED CONSTANTS ####
+
+fine_structure_constant = (2 * h * R_inf / (m_e * c)) ** 0.5 = α = alpha
+vacuum_permeability = 2 * α * h / (e ** 2 * c) = µ_0 = mu_0 = mu0 = magnetic_constant
+vacuum_permittivity = e ** 2 / (2 * α * h * c) = ε_0 = epsilon_0 = eps_0 = eps0 = electric_constant
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,3 @@
		### Unit definitions

		The unit definitions are currently sourced from the NIST 2018 [CODATA](https://physics.nist.gov/cuu/Constants/), but may be updated in future versions as new CODATA updates are made.