Update implicit solvent salt concentration/kappa handling. Now, if

implicitSolventKappa is provided it will be used. Otherwise,
implicitSolventSaltConc will be used to compute kappa from the solvent
dielectric constant and simulation temperature.

In order to provide a target temperature, an optional 'temperature' parameter
had to be added to the function call. The docstring makes it clear that
temperature is only used for the salt concentration -> debye length conversion.

wrappers/python/simtk/openmm/app/amberprmtopfile.py

@@ -144,6 +144,7 @@ class AmberPrmtopFile(object):
     def createSystem(self, nonbondedMethod=ff.NoCutoff, nonbondedCutoff=1.0*unit.nanometer,
                      constraints=None, rigidWater=True, implicitSolvent=None,
                      implicitSolventSaltConc=0.0*(unit.moles/unit.liter),
+                     implicitSolventKappa=None, temperature=298.15*unit.kelvin,
                      soluteDielectric=1.0, solventDielectric=78.5,
                      removeCMMotion=True, hydrogenMass=None, ewaldErrorTolerance=0.0005):
         """Construct an OpenMM System representing the topology described by this prmtop file.
@@ -157,7 +158,11 @@ class AmberPrmtopFile(object):
          - rigidWater (boolean=True) If true, water molecules will be fully rigid regardless of the value passed for the constraints argument
          - implicitSolvent (object=None) If not None, the implicit solvent model to use.  Allowed values are HCT, OBC1, OBC2, GBn, or GBn2.
          - implicitSolventSaltConc (float=0.0*unit.moles/unit.liter) The salt concentration for GB 
-                                             calculations (modelled as a debye screening parameter)
+                    calculations (modelled as a debye screening parameter). It is converted to the debye length (kappa)
+                    using the provided temperature and solventDielectric
+         - temperature (float=300*kelvin) Temperature of the system. Only used to compute the Debye length from
+                    implicitSolventSoltConc
+         - implicitSolventKappa (float units of 1/length) If this value is set, implicitSolventSaltConc will be ignored.
          - soluteDielectric (float=1.0) The solute dielectric constant to use in the implicit solvent model.
          - solventDielectric (float=78.5) The solvent dielectric constant to use in the implicit solvent model.
          - removeCMMotion (boolean=True) If true, a CMMotionRemover will be added to the System
@@ -199,20 +204,26 @@ class AmberPrmtopFile(object):
             implicitString = 'GBn2'
         else:
             raise ValueError('Illegal value for implicit solvent model')
-        # Use the conversion of ionic strength to kappa used by Amber (external
-        # dielectric of 78.5 -- it's unclear where the 0.10806 comes from
-        # though, although code comments in Amber say 'assuming M salt
-        # concentration, T=298.15, and epsext=78.5')
+        # If implicitSolventKappa is None, compute it from the salt concentration
+        if implicitSolvent is not None and implicitSolventKappa is None:
             if unit.is_quantity(implicitSolventSaltConc):
                 implicitSolventSaltConc = implicitSolventSaltConc.value_in_unit(unit.moles/unit.liter)
-        # 0.73 to account for ion exclusions. The Amber Debye screening length
-        # units will be in 1/angstrom given these units. Not a pretty way to do
-        # this...
-        implicitSolventKappa = 0.73 * sqrt(0.10806 * implicitSolventSaltConc) / unit.angstrom
-        sys = amber_file_parser.readAmberSystem(prmtop_loader=self._prmtop, shake=constraintString, nonbondedCutoff=nonbondedCutoff,
-                                                 nonbondedMethod=methodMap[nonbondedMethod], flexibleConstraints=False, gbmodel=implicitString,
-                                                 soluteDielectric=soluteDielectric, solventDielectric=solventDielectric, implicitSolventKappa=implicitSolventKappa,
+            # The constant is 1 / sqrt( epsilon_0 * kB / (2 * NA * q^2 * 1000) )
+            # where NA is avogadro's number, epsilon_0 is the permittivity of
+            # free space, q is the elementary charge (this number matches
+            # Amber's kappa conversion factor)
+            implicitSolventKappa = 50.33355 * sqrt(implicitSolventSaltConc / solventDielectric / temperature)
+            # Multiply by 0.73 to account for ion exclusions.
+            implicitSolventKappa *= 0.73
+        elif implicitSolvent is None:
+            implicitSolventKappa = 0.0
+
+        sys = amber_file_parser.readAmberSystem(prmtop_loader=self._prmtop, shake=constraintString,
+                        nonbondedCutoff=nonbondedCutoff, nonbondedMethod=methodMap[nonbondedMethod],
+                        flexibleConstraints=False, gbmodel=implicitString, soluteDielectric=soluteDielectric,
+                        solventDielectric=solventDielectric, implicitSolventKappa=implicitSolventKappa,
                         rigidWater=rigidWater, elements=self.elements)
+
         if hydrogenMass is not None:
             for atom1, atom2 in self.topology.bonds():
                 if atom1.element == elem.hydrogen: