From 2015b4e5edf5a9fa1ef8b044e4b6eb6cf2c8a945 Mon Sep 17 00:00:00 2001
From: "Matthew W. Thompson" <mattwthompson@protonmail.com>
Date: Thu, 24 Sep 2026 10:23:52 -0500
Subject: [PATCH 1/6] Stage tensor -> SMIRNOFF function

---
 tyff/_tests/convertors/openff/test_tensors.py | 103 ++++++++++++++++++
 tyff/converters/__init__.py                   |   2 +
 tyff/converters/openff/_tensors.py            |  37 +++++++
 3 files changed, 142 insertions(+)
 create mode 100644 tyff/_tests/convertors/openff/test_tensors.py
 create mode 100644 tyff/converters/openff/_tensors.py

diff --git a/tyff/_tests/convertors/openff/test_tensors.py b/tyff/_tests/convertors/openff/test_tensors.py
new file mode 100644
index 0000000..78b4626
--- /dev/null
+++ b/tyff/_tests/convertors/openff/test_tensors.py
@@ -0,0 +1,103 @@
+"""Test conversion of tensor force fields back to SMIRNOFF force fields."""
+
+import pytest
+import torch
+from openff.toolkit import ForceField, Molecule
+from openff.toolkit.typing.engines.smirnoff.parameters import VirtualSiteType
+
+import tyff.converters
+
+
+@pytest.fixture
+def sage():
+    return ForceField("openff-2.3.0.offxml")
+
+
+@pytest.fixture
+def sage_with_bond_charge():
+    sage = ForceField("openff-2.3.0.offxml")
+    sage.get_parameter_handler("VirtualSites")
+    sage["VirtualSites"].add_parameter(
+        parameter=VirtualSiteType(
+            smirks="[#6:2]-[#17X1:1]",
+            type="BondCharge",
+            match="all_permutations",
+            distance="0.8 * angstrom ** 1",
+            charge_increment1="0.123 * elementary_charge ** 1",
+            charge_increment2="0.0 * elementary_charge ** 1",
+        ),
+    )
+
+    return sage
+
+
+@pytest.fixture
+def ethanol():
+    return Molecule.from_smiles("CCO")
+
+
+@pytest.fixture
+def methyl_chloride():
+    return Molecule.from_smiles("CCl")
+
+
+def test_convert_no_modifications(ethanol, sage):
+    """
+    Test basic behavior of convert_tensor_force_field with no modifications of inputs.
+    """
+    interchange = sage.create_interchange(ethanol.to_topology())
+
+    tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
+
+    new_force_field = tyff.converters.convert_tensor_force_field(
+        sage,
+        tensor_force_field,
+    )
+
+    assert hash(new_force_field) == hash(sage)
+
+
+@pytest.mark.parametrize("handler_to_perturb", ["Bonds", "Angles", "ProperTorsions"])
+def test_convert_after_perturbation(ethanol, sage, handler_to_perturb):
+    """
+    Test that a tensor force field, randomly perturbed from the original SMIRNOFF source parameters, can be
+    converted back into a SMIRNOFF force field.
+    """
+
+    interchange = sage.create_interchange(ethanol.to_topology())
+
+    tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
+
+    # apply random perturbation to one element in on parameter tensor
+    shape = tensor_force_field.potentials_by_type[handler_to_perturb].parameters.shape
+    indicies_to_perturb = tuple(torch.randint(0, this_shape, (1,)).item() for this_shape in shape)
+
+    tensor_force_field.potentials_by_type[handler_to_perturb].parameters[indicies_to_perturb] *= 1.432
+
+    new_force_field = tyff.converters.convert_tensor_force_field(
+        sage,
+        tensor_force_field,
+    )
+
+    assert hash(new_force_field) != hash(sage)
+
+
+def test_convert_vsites(methyl_chloride, sage_with_bond_charge):
+    """
+    Test that a tensor force field, modified from original SMIRNOFF source parameters, can be
+    converted back into a SMIRNOFF force field.
+    """
+
+    interchange = sage_with_bond_charge.create_interchange(methyl_chloride.to_topology())
+
+    tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
+
+    # modify tensor force field
+
+    new_force_field = tyff.converters.convert_tensor_force_field(
+        sage_with_bond_charge,
+        tensor_force_field,
+    )
+
+    # need to assert this is NOT the case
+    assert hash(new_force_field) == hash(sage_with_bond_charge)
diff --git a/tyff/converters/__init__.py b/tyff/converters/__init__.py
index 88213b9..82bfd1a 100644
--- a/tyff/converters/__init__.py
+++ b/tyff/converters/__init__.py
@@ -5,6 +5,7 @@
     convert_interchange,
     smirnoff_parameter_converter,
 )
+from tyff.converters.openff._tensors import convert_tensor_force_field
 from tyff.converters.openmm import (
     convert_to_openmm_ffxml,
     convert_to_openmm_force,
@@ -16,6 +17,7 @@
 __all__ = [
     "convert_handlers",
     "convert_interchange",
+    "convert_tensor_force_field",
     "convert_to_openmm_ffxml",
     "convert_to_openmm_force",
     "convert_to_openmm_system",
diff --git a/tyff/converters/openff/_tensors.py b/tyff/converters/openff/_tensors.py
new file mode 100644
index 0000000..77b1e72
--- /dev/null
+++ b/tyff/converters/openff/_tensors.py
@@ -0,0 +1,37 @@
+import openff.interchange
+import openff.toolkit
+
+import tyff
+
+
+def convert_tensor_force_field(
+    original_force_field: openff.toolkit.ForceField,
+    tensor_force_field: tyff.TensorForceField,
+) -> openff.toolkit.ForceField:
+    """
+    Convert a tensor force field back into a SMIRNOFF force field, using the
+    original force field as a template. Electrostatics / applied partial
+    charges are not updated.
+    """
+    import copy
+
+    updated = copy.deepcopy(original_force_field)
+
+    for potential in tensor_force_field.potentials:
+        if potential.type == "Electrostatics":
+            continue
+
+        if potential.type == "ProperTorsions":
+            # TODO: implement this
+            continue
+
+        name = str(getattr(potential.type, "value", potential.type))
+        handler = updated.get_parameter_handler(name)
+
+        for row, key in enumerate(potential.parameter_keys):
+            for index, col in enumerate(potential.parameter_cols):
+                value = potential.parameters[row, index].item()
+
+                setattr(handler[key.id], col, value * potential.parameter_units[index])
+
+    return updated

From a07cafee47e2512ccc1ab8236827d42da3c07852 Mon Sep 17 00:00:00 2001
From: "Matthew W. Thompson" <mattwthompson@protonmail.com>
Date: Thu, 24 Sep 2026 12:02:56 -0500
Subject: [PATCH 2/6] Update test

---
 tyff/_tests/convertors/openff/test_tensors.py | 56 +++++++++++++++----
 tyff/converters/openff/_tensors.py            |  9 +--
 2 files changed, 47 insertions(+), 18 deletions(-)

diff --git a/tyff/_tests/convertors/openff/test_tensors.py b/tyff/_tests/convertors/openff/test_tensors.py
index 78b4626..3e656aa 100644
--- a/tyff/_tests/convertors/openff/test_tensors.py
+++ b/tyff/_tests/convertors/openff/test_tensors.py
@@ -1,7 +1,8 @@
 """Test conversion of tensor force fields back to SMIRNOFF force fields."""
 
+import random
+
 import pytest
-import torch
 from openff.toolkit import ForceField, Molecule
 from openff.toolkit.typing.engines.smirnoff.parameters import VirtualSiteType
 
@@ -32,8 +33,8 @@ def sage_with_bond_charge():
 
 
 @pytest.fixture
-def ethanol():
-    return Molecule.from_smiles("CCO")
+def phenol():
+    return Molecule.from_smiles("c1ccccc1O")
 
 
 @pytest.fixture
@@ -41,11 +42,11 @@ def methyl_chloride():
     return Molecule.from_smiles("CCl")
 
 
-def test_convert_no_modifications(ethanol, sage):
+def test_convert_no_modifications(phenol, sage):
     """
     Test basic behavior of convert_tensor_force_field with no modifications of inputs.
     """
-    interchange = sage.create_interchange(ethanol.to_topology())
+    interchange = sage.create_interchange(phenol.to_topology())
 
     tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
 
@@ -57,22 +58,28 @@ def test_convert_no_modifications(ethanol, sage):
     assert hash(new_force_field) == hash(sage)
 
 
-@pytest.mark.parametrize("handler_to_perturb", ["Bonds", "Angles", "ProperTorsions"])
-def test_convert_after_perturbation(ethanol, sage, handler_to_perturb):
+@pytest.mark.parametrize(
+    "handler_to_perturb,column_index",
+    [
+        ("Bonds", 0),  # k, length
+        ("Angles", 0),  # k, angle
+        ("ProperTorsions", 0),  # k, etc.
+        ("ImproperTorsions", 0),  # k, etc.
+    ],
+)
+def test_convert_after_perturbation(phenol, sage, handler_to_perturb, column_index):
     """
     Test that a tensor force field, randomly perturbed from the original SMIRNOFF source parameters, can be
     converted back into a SMIRNOFF force field.
     """
+    factor = random.random()
 
-    interchange = sage.create_interchange(ethanol.to_topology())
+    interchange = sage.create_interchange(phenol.to_topology())
 
     tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
 
     # apply random perturbation to one element in on parameter tensor
-    shape = tensor_force_field.potentials_by_type[handler_to_perturb].parameters.shape
-    indicies_to_perturb = tuple(torch.randint(0, this_shape, (1,)).item() for this_shape in shape)
-
-    tensor_force_field.potentials_by_type[handler_to_perturb].parameters[indicies_to_perturb] *= 1.432
+    tensor_force_field.potentials_by_type[handler_to_perturb].parameters[:, column_index] *= factor
 
     new_force_field = tyff.converters.convert_tensor_force_field(
         sage,
@@ -81,7 +88,32 @@ def test_convert_after_perturbation(ethanol, sage, handler_to_perturb):
 
     assert hash(new_force_field) != hash(sage)
 
+    modified_keys = [key.id for key in tensor_force_field.potentials_by_type[handler_to_perturb].parameter_keys]
+
+    if handler_to_perturb in ("ProperTorsions", "ImproperTorsions"):
+        # parameter k values are list, so need to gather differently
+        found_factors = [
+            (a / b).m_as("dimensionless")
+            for a, b in zip(
+                new_force_field[handler_to_perturb][modified_keys[-1]].k, sage[handler_to_perturb][modified_keys[-1]].k
+            )
+        ]
+
+    else:
+        found_factors = [
+            (
+                new_force_field[handler_to_perturb][modified_keys[-1]].k
+                / sage[handler_to_perturb][modified_keys[-1]].k
+            ).m_as("dimensionless")
+        ]
+
+    for value in found_factors:
+        assert value == pytest.approx(factor), (
+            f"Expected k to be scaled by {factor}, but found to be scaled by {value}"
+        )
+
 
+@pytest.mark.skip(reason="not yet implemented")
 def test_convert_vsites(methyl_chloride, sage_with_bond_charge):
     """
     Test that a tensor force field, modified from original SMIRNOFF source parameters, can be
diff --git a/tyff/converters/openff/_tensors.py b/tyff/converters/openff/_tensors.py
index 77b1e72..a50b549 100644
--- a/tyff/converters/openff/_tensors.py
+++ b/tyff/converters/openff/_tensors.py
@@ -18,11 +18,7 @@ def convert_tensor_force_field(
     updated = copy.deepcopy(original_force_field)
 
     for potential in tensor_force_field.potentials:
-        if potential.type == "Electrostatics":
-            continue
-
-        if potential.type == "ProperTorsions":
-            # TODO: implement this
+        if potential.type in ("Electrostatics", "ProperTorsions", "ImproperTorsions"):
             continue
 
         name = str(getattr(potential.type, "value", potential.type))
@@ -31,7 +27,8 @@ def convert_tensor_force_field(
         for row, key in enumerate(potential.parameter_keys):
             for index, col in enumerate(potential.parameter_cols):
                 value = potential.parameters[row, index].item()
-
+                # skip if parameter is already set to this value?
+                # maybe unnecessarily slow ...
                 setattr(handler[key.id], col, value * potential.parameter_units[index])
 
     return updated

From 412338a46b06a39ae66d8e150d362c555018fc2d Mon Sep 17 00:00:00 2001
From: "Matthew W. Thompson" <mattwthompson@protonmail.com>
Date: Thu, 24 Sep 2026 13:29:42 -0500
Subject: [PATCH 3/6] Enable updating force constants when `k` is list-like

---
 tyff/_tests/convertors/openff/test_tensors.py | 54 +++++++++++++++++--
 tyff/converters/openff/_tensors.py            | 32 ++++++++---
 2 files changed, 77 insertions(+), 9 deletions(-)

diff --git a/tyff/_tests/convertors/openff/test_tensors.py b/tyff/_tests/convertors/openff/test_tensors.py
index 3e656aa..f8ba228 100644
--- a/tyff/_tests/convertors/openff/test_tensors.py
+++ b/tyff/_tests/convertors/openff/test_tensors.py
@@ -9,6 +9,19 @@
 import tyff.converters
 
 
+def tidy_force_field(force_field: ForceField) -> ForceField:
+    # work around vdWType squishiness when comparing force fields via hash
+    # https://github.com/openforcefield/openff-toolkit/issues/2241
+    for parameter in force_field["vdW"].parameters:
+        parameter.rmin_half = round(parameter.rmin_half, 12)
+
+    return force_field
+
+
+def force_fields_are_equal(force_field_1: ForceField, force_field_2: ForceField) -> bool:
+    return hash(tidy_force_field(force_field_1)) == hash(tidy_force_field(force_field_2))
+
+
 @pytest.fixture
 def sage():
     return ForceField("openff-2.3.0.offxml")
@@ -42,6 +55,12 @@ def methyl_chloride():
     return Molecule.from_smiles("CCl")
 
 
+@pytest.fixture
+def methyl_phenyl_disulfide():
+    # need a molecule with layered torsions and impropers
+    return Molecule.from_smiles("c1ccccc1(SSC)")
+
+
 def test_convert_no_modifications(phenol, sage):
     """
     Test basic behavior of convert_tensor_force_field with no modifications of inputs.
@@ -55,7 +74,15 @@ def test_convert_no_modifications(phenol, sage):
         tensor_force_field,
     )
 
-    assert hash(new_force_field) == hash(sage)
+    try:
+        force_fields_are_equal(new_force_field, sage)
+    except AssertionError as error:
+        import copy
+
+        new_force_field.to_file("new.offxml")
+        copy.deepcopy(sage).to_file("old.offxml")
+
+        raise error
 
 
 @pytest.mark.parametrize(
@@ -67,14 +94,14 @@ def test_convert_no_modifications(phenol, sage):
         ("ImproperTorsions", 0),  # k, etc.
     ],
 )
-def test_convert_after_perturbation(phenol, sage, handler_to_perturb, column_index):
+def test_convert_after_perturbation(methyl_phenyl_disulfide, sage, handler_to_perturb, column_index):
     """
     Test that a tensor force field, randomly perturbed from the original SMIRNOFF source parameters, can be
     converted back into a SMIRNOFF force field.
     """
     factor = random.random()
 
-    interchange = sage.create_interchange(phenol.to_topology())
+    interchange = sage.create_interchange(methyl_phenyl_disulfide.to_topology())
 
     tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
 
@@ -113,6 +140,27 @@ def test_convert_after_perturbation(phenol, sage, handler_to_perturb, column_ind
         )
 
 
+def test_convert_vsites_unsupported(methyl_chloride, sage_with_bond_charge):
+    """
+    Test that an error is raised when attempting to convert back a tensor force field
+    which contatins virtual site parameters.
+
+    Remove this test when the feature is implemented.
+    """
+
+    interchange = sage_with_bond_charge.create_interchange(methyl_chloride.to_topology())
+
+    tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
+
+    # modify tensor force field
+
+    with pytest.raises(NotImplementedError):
+        tyff.converters.convert_tensor_force_field(
+            sage_with_bond_charge,
+            tensor_force_field,
+        )
+
+
 @pytest.mark.skip(reason="not yet implemented")
 def test_convert_vsites(methyl_chloride, sage_with_bond_charge):
     """
diff --git a/tyff/converters/openff/_tensors.py b/tyff/converters/openff/_tensors.py
index a50b549..94361a8 100644
--- a/tyff/converters/openff/_tensors.py
+++ b/tyff/converters/openff/_tensors.py
@@ -1,5 +1,6 @@
 import openff.interchange
 import openff.toolkit
+from openff.toolkit.utils.exceptions import ParameterLookupError
 
 import tyff
 
@@ -10,25 +11,44 @@ def convert_tensor_force_field(
 ) -> openff.toolkit.ForceField:
     """
     Convert a tensor force field back into a SMIRNOFF force field, using the
-    original force field as a template. Electrostatics / applied partial
-    charges are not updated.
+    original force field as a template.
+
+    Electrostatics / applied partial charges are not updated.
+
+    `ImproperTorsionType.idivf` values are ignored.
+
+    Virtual site parameters are not (yet) supported.
     """
     import copy
 
     updated = copy.deepcopy(original_force_field)
 
     for potential in tensor_force_field.potentials:
-        if potential.type in ("Electrostatics", "ProperTorsions", "ImproperTorsions"):
+        if potential.type in ("Electrostatics"):
             continue
 
+        if potential.type in ("VirtualSites"):
+            raise NotImplementedError("Virtual site parameters are not yet supported.")
+
         name = str(getattr(potential.type, "value", potential.type))
         handler = updated.get_parameter_handler(name)
 
         for row, key in enumerate(potential.parameter_keys):
             for index, col in enumerate(potential.parameter_cols):
+                if col == "idivf":
+                    continue
+
                 value = potential.parameters[row, index].item()
-                # skip if parameter is already set to this value?
-                # maybe unnecessarily slow ...
-                setattr(handler[key.id], col, value * potential.parameter_units[index])
+                try:
+                    # skip if parameter is already set to this value?
+                    # maybe unnecessarily slow ...
+                    setattr(handler[key.id], col, value * potential.parameter_units[index])
+                except TypeError:
+                    # can't set a list of k with just one k value, but can directly set the element
+                    # of this parameter's k-list
+                    getattr(handler[key.id], col)[key.mult] = value * potential.parameter_units[index]
+                except ParameterLookupError as error:
+                    # this is probably a failure to look up a virtual site's vdW parameter
+                    raise NotImplementedError("Virtual site parameters are not yet supported.") from error
 
     return updated

From fe1a80add3b2b63f19e60d7d47de6f1d4a32c6b0 Mon Sep 17 00:00:00 2001
From: "Matthew W. Thompson" <mattwthompson@protonmail.com>
Date: Thu, 24 Sep 2026 16:14:44 -0500
Subject: [PATCH 4/6] Test vdW modification, fix force field comparison and
 no-op

---
 tyff/_tests/convertors/openff/test_tensors.py | 86 +++++++++++++++----
 1 file changed, 67 insertions(+), 19 deletions(-)

diff --git a/tyff/_tests/convertors/openff/test_tensors.py b/tyff/_tests/convertors/openff/test_tensors.py
index f8ba228..5c90d91 100644
--- a/tyff/_tests/convertors/openff/test_tensors.py
+++ b/tyff/_tests/convertors/openff/test_tensors.py
@@ -1,5 +1,6 @@
 """Test conversion of tensor force fields back to SMIRNOFF force fields."""
 
+import math
 import random
 
 import pytest
@@ -10,11 +11,27 @@
 
 
 def tidy_force_field(force_field: ForceField) -> ForceField:
+    """Ensure most parameters are expressed in consistent units, to be used before hashing."""
     # work around vdWType squishiness when comparing force fields via hash
     # https://github.com/openforcefield/openff-toolkit/issues/2241
     for parameter in force_field["vdW"].parameters:
         parameter.rmin_half = round(parameter.rmin_half, 12)
 
+    for parameter in force_field["Bonds"].parameters:
+        parameter.k = parameter.k.to("angstrom ** -2 * kilocalorie ** 1 * mole ** -1")
+
+    for parameter in force_field["Angles"].parameters:
+        parameter.angle = parameter.angle.to("radian")
+        parameter.k = parameter.k.to("kilocalorie ** 1 * mole ** -1 * radian ** -2")
+
+    for parameter in force_field["ProperTorsions"].parameters:
+        parameter.phase = [phase.to("radian") for phase in parameter.phase]
+        parameter.k = [k.to("kilocalorie ** 1 * mole ** -1 * radian ** -2") for k in parameter.k]
+
+    for parameter in force_field["ImproperTorsions"].parameters:
+        parameter.phase = [phase.to("radian") for phase in parameter.phase]
+        parameter.k = [k.to("kilocalorie ** 1 * mole ** -1 * radian ** -2") for k in parameter.k]
+
     return force_field
 
 
@@ -75,38 +92,57 @@ def test_convert_no_modifications(phenol, sage):
     )
 
     try:
-        force_fields_are_equal(new_force_field, sage)
+        assert force_fields_are_equal(new_force_field, sage)
     except AssertionError as error:
         import copy
 
-        new_force_field.to_file("new.offxml")
-        copy.deepcopy(sage).to_file("old.offxml")
+        tidy_force_field(new_force_field).to_file("new.offxml")
+        copy.deepcopy(tidy_force_field(sage)).to_file("old.offxml")
 
         raise error
 
 
 @pytest.mark.parametrize(
-    "handler_to_perturb,column_index",
+    "handler_to_perturb,column_index,column_name",
     [
-        ("Bonds", 0),  # k, length
-        ("Angles", 0),  # k, angle
-        ("ProperTorsions", 0),  # k, etc.
-        ("ImproperTorsions", 0),  # k, etc.
+        ("vdW", 0, "epsilon"),
+        ("vdW", 1, "sigma"),
+        ("Bonds", 0, "k"),
+        ("Bonds", 1, "length"),
+        ("Angles", 0, "k"),
+        ("Angles", 1, "angle"),
+        # skip periodicity and idivf with torsions
+        (
+            "ProperTorsions",
+            0,
+            "k",
+        ),
+        ("ProperTorsions", 2, "phase"),  # angle for proper torsions
+        ("ImproperTorsions", 0, "k"),  # k, etc.
+        ("ImproperTorsions", 2, "phase"),  # angle for improper torsions
     ],
 )
-def test_convert_after_perturbation(methyl_phenyl_disulfide, sage, handler_to_perturb, column_index):
+def test_convert_after_perturbation(methyl_phenyl_disulfide, sage, handler_to_perturb, column_index, column_name):
     """
     Test that a tensor force field, randomly perturbed from the original SMIRNOFF source parameters, can be
     converted back into a SMIRNOFF force field.
     """
-    factor = random.random()
-
     interchange = sage.create_interchange(methyl_phenyl_disulfide.to_topology())
 
     tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
 
     # apply random perturbation to one element in on parameter tensor
-    tensor_force_field.potentials_by_type[handler_to_perturb].parameters[:, column_index] *= factor
+    if column_name != "phase":
+        factor = 1 + 0.5 * random.random()
+
+        tensor_force_field.potentials_by_type[handler_to_perturb].parameters[:, column_index] *= factor
+
+    else:
+        # this is more of an offset than a coefficient, since phase is often 0
+        factor = math.pi / 6
+
+        tensor_force_field.potentials_by_type[handler_to_perturb].parameters[:, column_index] += factor
+        print(tensor_force_field.potentials_by_type[handler_to_perturb].parameters[:, column_index], factor)
 
     new_force_field = tyff.converters.convert_tensor_force_field(
         sage,
@@ -119,18 +155,30 @@ def test_convert_after_perturbation(methyl_phenyl_disulfide, sage, handler_to_pe
 
     if handler_to_perturb in ("ProperTorsions", "ImproperTorsions"):
         # parameter k values are list, so need to gather differently
-        found_factors = [
-            (a / b).m_as("dimensionless")
+        if column_name == "k":
+            found_factors = [
+                (a / b).m_as("dimensionless")
+                for a, b in zip(
+                    getattr(new_force_field[handler_to_perturb][modified_keys[-1]], column_name),
+                    getattr(sage[handler_to_perturb][modified_keys[-1]], column_name),
+                )
+            ]
+        elif column_name == "phase":
+            # special case of offset instead of multipliciative perturbation
             for a, b in zip(
-                new_force_field[handler_to_perturb][modified_keys[-1]].k, sage[handler_to_perturb][modified_keys[-1]].k
-            )
-        ]
+                getattr(new_force_field[handler_to_perturb][modified_keys[-1]], column_name),
+                getattr(sage[handler_to_perturb][modified_keys[-1]], column_name),
+            ):
+                assert ((a - b).m_as("radian")) % math.pi == pytest.approx(factor)
+
+            # exit early for this special case
+            return
 
     else:
         found_factors = [
             (
-                new_force_field[handler_to_perturb][modified_keys[-1]].k
-                / sage[handler_to_perturb][modified_keys[-1]].k
+                getattr(new_force_field[handler_to_perturb][modified_keys[-1]], column_name)
+                / getattr(sage[handler_to_perturb][modified_keys[-1]], column_name)
             ).m_as("dimensionless")
         ]
 

From b84d1d28d460bc49cac15c7646169ace218f62bc Mon Sep 17 00:00:00 2001
From: "Matthew W. Thompson" <mattwthompson@protonmail.com>
Date: Thu, 24 Sep 2026 16:25:34 -0500
Subject: [PATCH 5/6] Suggestsions from Claude

---
 tyff/_tests/convertors/openff/test_tensors.py | 33 +++++++++++++++++++
 tyff/converters/openff/_tensors.py            |  6 ++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/tyff/_tests/convertors/openff/test_tensors.py b/tyff/_tests/convertors/openff/test_tensors.py
index 5c90d91..6b10942 100644
--- a/tyff/_tests/convertors/openff/test_tensors.py
+++ b/tyff/_tests/convertors/openff/test_tensors.py
@@ -4,6 +4,7 @@
 import random
 
 import pytest
+import torch
 from openff.toolkit import ForceField, Molecule
 from openff.toolkit.typing.engines.smirnoff.parameters import VirtualSiteType
 
@@ -39,6 +40,21 @@ def force_fields_are_equal(force_field_1: ForceField, force_field_2: ForceField)
     return hash(tidy_force_field(force_field_1)) == hash(tidy_force_field(force_field_2))
 
 
+def tensor_force_fields_are_equal(
+    tensor_force_field_1: tyff.TensorForceField, tensor_force_field_2: tyff.TensorForceField
+) -> bool:
+    assert tensor_force_field_1.potentials_by_type.keys() == tensor_force_field_2.potentials_by_type.keys()
+
+    for potential1, potential2 in zip(
+        tensor_force_field_1.potentials,
+        tensor_force_field_2.potentials,
+    ):
+        if not torch.allclose(potential1.parameters, potential2.parameters):
+            return False
+
+    return True
+
+
 @pytest.fixture
 def sage():
     return ForceField("openff-2.3.0.offxml")
@@ -102,6 +118,23 @@ def test_convert_no_modifications(phenol, sage):
         raise error
 
 
+def test_double_roundtrip(phenol, sage):
+    interchange = sage.create_interchange(phenol.to_topology())
+
+    tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
+
+    new_force_field = tyff.converters.convert_tensor_force_field(
+        sage,
+        tensor_force_field,
+    )
+
+    new_tensor_force_field, _ = tyff.converters.convert_interchange(
+        new_force_field.create_interchange(phenol.to_topology())
+    )
+
+    assert tensor_force_fields_are_equal(tensor_force_field, new_tensor_force_field)
+
+
 @pytest.mark.parametrize(
     "handler_to_perturb,column_index,column_name",
     [
diff --git a/tyff/converters/openff/_tensors.py b/tyff/converters/openff/_tensors.py
index 94361a8..239ef6c 100644
--- a/tyff/converters/openff/_tensors.py
+++ b/tyff/converters/openff/_tensors.py
@@ -13,6 +13,8 @@ def convert_tensor_force_field(
     Convert a tensor force field back into a SMIRNOFF force field, using the
     original force field as a template.
 
+    Does not look at handler-level attributes (1-4 scaling factors, cutoffs, etc.).
+
     Electrostatics / applied partial charges are not updated.
 
     `ImproperTorsionType.idivf` values are ignored.
@@ -24,10 +26,10 @@ def convert_tensor_force_field(
     updated = copy.deepcopy(original_force_field)
 
     for potential in tensor_force_field.potentials:
-        if potential.type in ("Electrostatics"):
+        if potential.type in ("Electrostatics",):
             continue
 
-        if potential.type in ("VirtualSites"):
+        if potential.type in ("VirtualSites",):
             raise NotImplementedError("Virtual site parameters are not yet supported.")
 
         name = str(getattr(potential.type, "value", potential.type))

From debfbf0bad9fd8794032799f74b5f2fff6f791eb Mon Sep 17 00:00:00 2001
From: Matt Thompson <matt.thompson@openforcefield.org>
Date: Thu, 24 Sep 2026 16:48:26 -0500
Subject: [PATCH 6/6] Fix comment and clean up debug output in tests

Corrected comment for clarity and removed debug print statement.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
---
 tyff/_tests/convertors/openff/test_tensors.py | 3 +--
 tyff/converters/openff/_tensors.py            | 1 -
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/tyff/_tests/convertors/openff/test_tensors.py b/tyff/_tests/convertors/openff/test_tensors.py
index 6b10942..4b50413 100644
--- a/tyff/_tests/convertors/openff/test_tensors.py
+++ b/tyff/_tests/convertors/openff/test_tensors.py
@@ -164,7 +164,7 @@ def test_convert_after_perturbation(methyl_phenyl_disulfide, sage, handler_to_pe
 
     tensor_force_field, _ = tyff.converters.convert_interchange(interchange)
 
-    # apply random perturbation to one element in on parameter tensor
+    # apply random perturbation to one element in one parameter tensor
     if column_name != "phase":
         factor = 1 + 0.5 * random.random()
 
@@ -175,7 +175,6 @@ def test_convert_after_perturbation(methyl_phenyl_disulfide, sage, handler_to_pe
         factor = math.pi / 6
 
         tensor_force_field.potentials_by_type[handler_to_perturb].parameters[:, column_index] += factor
-        print(tensor_force_field.potentials_by_type[handler_to_perturb].parameters[:, column_index], factor)
 
     new_force_field = tyff.converters.convert_tensor_force_field(
         sage,
diff --git a/tyff/converters/openff/_tensors.py b/tyff/converters/openff/_tensors.py
index 239ef6c..468ab37 100644
--- a/tyff/converters/openff/_tensors.py
+++ b/tyff/converters/openff/_tensors.py
@@ -1,4 +1,3 @@
-import openff.interchange
 import openff.toolkit
 from openff.toolkit.utils.exceptions import ParameterLookupError