CHARMM36 supports disulfide bonds

wrappers/python/simtk/openmm/app/data/charmm36.xml

@@ -96340,7 +96340,16 @@
       <RemoveBond atomName1="PX" atomName2="O1X"/>
       <RemoveBond atomName1="PX" atomName2="O2X"/>
     </Patch>
-  </Patches>
+    <Patch name="DISU" residues="2">
+      <RemoveAtom name="1:HG1"/>
+      <RemoveAtom name="2:HG1"/>
+      <ChangeAtom charge="-0.1" name="CB" type="CT2"/>
+      <ChangeAtom charge="-0.08" name="SG" type="SM"/>
+      <AddBond atomName1="1:SG" atomName2="2:SG"/>
+      <ApplyToResidue name="1:CYS"/>
+      <ApplyToResidue name="2:CYS"/>
+    </Patch>
+</Patches>
   <HarmonicBondForce>
     <Bond k="200831.99999999997" length="0.14550000000000002" type1="NH2" type2="CT1"/>
     <Bond k="255223.99999999994" length="0.1375" type1="CA" type2="CAI"/>

wrappers/python/tests/TestForceField.py

@@ -845,6 +845,14 @@ class TestForceField(unittest.TestCase):
         self.assertEqual(system1_indexes, [51, 56, 54, 55])
         self.assertEqual(system2_indexes, [51, 55, 54, 56])
 
+    def test_Disulfides(self):
+        """Test that various force fields handle disulfides correctly."""
+        pdb = PDBFile('systems/bpti.pdb')
+        for ff in ['amber99sb.xml', 'amber14-all.xml', 'charmm36.xml', 'amberfb15.xml', 'amoeba2013.xml']:
+            forcefield = ForceField(ff)
+            system = forcefield.createSystem(pdb.topology)
+
+
 class AmoebaTestForceField(unittest.TestCase):
     """Test the ForceField.createSystem() method with the AMOEBA forcefield."""