Fixes rmin_half / sigma factor conversion (#807).

openforcefield · Jan 8, 2021 · 2f6bc3f · 2f6bc3f
1 parent 988c6d6
commit 2f6bc3f
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diff --git a/docs/releasehistory.rst b/docs/releasehistory.rst
@@ -15,6 +15,10 @@ New features
 - `PR #794 <https://github.com/openforcefield/openforcefield/pull/974>`_: Adds a decorator
   ``@requires_package`` that denotes a function requires an optional dependency.
 
+Bugfixes
+""""""""
+- `PR #808 <https://github.com/openforcefield/openforcefield/pull/808>`_: Fixes the interconversion
+  between a vdW ``sigma`` and ``rmin_half`` parameter.
 
 0.8.2 - Bugfix release
 ----------------------

diff --git a/openforcefield/tests/test_parameters.py b/openforcefield/tests/test_parameters.py
@@ -1764,26 +1764,32 @@ def test_sigma_rmin_half(self):
 
         data = {
             "smirks": "[*:1]",
-            "sigma": 0.5 * unit.angstrom,
+            "rmin_half": 0.5 * unit.angstrom,
             "epsilon": 0.5 * unit.kilocalorie_per_mole,
         }
         param = vdWHandler.vdWType(**data)
 
         assert param.sigma is not None
         assert param.rmin_half is not None
-        assert param.sigma == param.rmin_half / 2 ** (1 / 6)
-        assert "sigma" in param.to_dict()
-        assert "rmin_half" not in param.to_dict()
+        assert numpy.isclose(
+            param.sigma.value_in_unit(unit.angstrom),
+            (2.0 * param.rmin_half / 2 ** (1 / 6)).value_in_unit(unit.angstrom),
+        )
+        assert "sigma" not in param.to_dict()
+        assert "rmin_half" in param.to_dict()
 
-        param.sigma = 0.8 * unit.angstrom
+        param.sigma = param.sigma
 
-        assert param.sigma == param.rmin_half / 2 ** (1 / 6)
+        assert numpy.isclose(param.rmin_half.value_in_unit(unit.angstrom), 0.5)
         assert "sigma" in param.to_dict()
         assert "rmin_half" not in param.to_dict()
 
-        param.rmin_half = 0.8 * unit.angstrom
+        param.rmin_half = param.rmin_half
 
-        assert param.sigma == param.rmin_half / 2 ** (1 / 6)
+        assert numpy.isclose(
+            param.sigma.value_in_unit(unit.angstrom),
+            (2.0 * param.rmin_half / 2 ** (1 / 6)).value_in_unit(unit.angstrom),
+        )
         assert "sigma" not in param.to_dict()
         assert "rmin_half" in param.to_dict()
 

diff --git a/openforcefield/typing/engines/smirnoff/parameters.py b/openforcefield/typing/engines/smirnoff/parameters.py
@@ -3513,13 +3513,13 @@ def __setattr__(self, name, value):
             if name == "rmin_half":
                 if type(value) == str:
                     value = object_to_quantity(value)
-                super().__setattr__("sigma", value / 2 ** (1 / 6))
+                super().__setattr__("sigma", 2.0 * value / 2 ** (1 / 6))
                 self._extra_nb_var = "sigma"
 
             if name == "sigma":
                 if type(value) == str:
                     value = object_to_quantity(value)
-                super().__setattr__("rmin_half", value * 2 ** (1 / 6))
+                super().__setattr__("rmin_half", value * 2 ** (1 / 6) / 2.0)
                 self._extra_nb_var = "rmin_half"
 
         def to_dict(