Add OPC and OPC3 ion sets (#3819)

* copy opc ions and opc*standard.xml from https://github.com/openmm/openmmforcefields/commit/b12c2e871b60275a10b3f8f56dfc2f6d5447591f

* auto-merged files

* move charges to residue definition

* remove unmerged files

* add docs

* rename U to U4+

* rename Ag+, Tl+, Gd3+ and I- residue names

* fix O-H bond length typo

* add opc3 constraints test

docs-source/usersguide/application/02_running_sims.rst

@@ -505,6 +505,8 @@ File                                 Parameters
 :file:`amber14/tip4pew.xml`          TIP4P-Ew water model\ :cite:`Horn2004` and ions
 :file:`amber14/tip4pfb.xml`          TIP4P-FB water model\ :cite:`Wang2014` and ions
 :file:`amber14/spce.xml`             SPC/E water model\ :cite:`Berendsen1987` and ions
+:code:`amber14/opc.xml`              OPC water model\ :cite:`Izadi2014` and ions
+:code:`amber14/opc3.xml`             OPC3 water model\ :cite:`Izadi2016` and ions
 ===================================  ============================================
 
 As a convenience, the file :file:`amber14-all.xml` can be used as a shortcut to include

wrappers/python/openmm/app/data/opc3.xml

@@ -17,7 +17,7 @@
   </Residue>
  </Residues>
  <HarmonicBondForce>
-  <Bond class1="OW" class2="HW" length="0.978882" k="502416.0"/>
+  <Bond class1="OW" class2="HW" length="0.0978882" k="502416.0"/>
  </HarmonicBondForce>
  <HarmonicAngleForce>
   <!-- angle = arccos(1 - (1.598507**2 / (2*0.978882**2))) -->

wrappers/python/tests/TestForceField.py

@@ -1325,6 +1325,13 @@ self.scriptExecuted = True
         # -2532.4862082354407
         self.assertTrue(abs(energy1 - energy_amber) < energy_tolerance)
 
+        context.applyConstraints(1e-12)
+        state = context.getState(getEnergy=True)
+        energy2 = state.getPotentialEnergy().value_in_unit(kilocalorie_per_mole)
+        self.assertTrue(abs(energy1 - energy_amber) < energy_tolerance)
+        self.assertTrue(abs(energy1 - energy2) < energy_tolerance)
+
+
 class AmoebaTestForceField(unittest.TestCase):
     """Test the ForceField.createSystem() method with the AMOEBA forcefield."""