Add SWM4-NDP water to modeller.addSolvent() (#3760)

* add swm4-ndp water to addSolvent

* add equilibrated box

* typo of water radius

* fixed addSolvent description

* add swm4 to tests

wrappers/python/openmm/app/modeller.py

@@ -416,7 +416,7 @@ class Modeller(object):
         forcefield : ForceField
             the ForceField to use for determining van der Waals radii and atomic charges
         model : str='tip3p'
-            the water model to use.  Supported values are 'tip3p', 'spce', 'tip4pew', and 'tip5p'. 
+            the water model to use.  Supported values are 'tip3p', 'spce', 'tip4pew', 'tip5p', and 'swm4ndp' (polarizable).
         boxSize : Vec3=None
             the size of the box to fill with water
         boxVectors : tuple of Vec3=None
@@ -454,6 +454,8 @@ class Modeller(object):
             waterRadius = 0.315365*vdwRadiusPerSigma
         elif model == 'tip5p':
             waterRadius = 0.312*vdwRadiusPerSigma
+        elif model == 'swm4ndp':
+            waterRadius = 0.318395*vdwRadiusPerSigma
         else:
             raise ValueError('Unknown water model: %s' % model)
         pdb = PDBFile(os.path.join(os.path.dirname(__file__), 'data', model+'.pdb'))

wrappers/python/tests/TestModeller.py

@@ -264,7 +264,7 @@ class TestModeller(unittest.TestCase):
 
         topology_start = self.pdb.topology
         topology_start.setUnitCellDimensions(Vec3(3.5, 3.5, 3.5)*nanometers)
-        for model in ['tip3p', 'spce', 'tip4pew', 'tip5p']:
+        for model in ['tip3p', 'spce', 'tip4pew', 'tip5p', 'swm4ndp']:
             forcefield = ForceField('amber10.xml', model + '.xml')
             modeller = Modeller(topology_start, self.positions)
             # delete water to get the "before" topology