Correct treatment of kappa when kappa == 0 and when there is a cutoff.

wrappers/python/simtk/openmm/app/internal/customgbforces.py

@@ -206,11 +206,19 @@ def _createEnergyTerms(force, SA, cutoff, kappa):
     elif SA is not None:
         raise ValueError('Unknown surface area method: '+SA)
     if cutoff is None:
+        if kappa > 0:
             force.addEnergyTerm("-138.935485*(1/soluteDielectric-exp(-kappa*f)/solventDielectric)*q1*q2/f;"
                                 "f=sqrt(r^2+B1*B2*exp(-r^2/(4*B1*B2)))", CustomGBForce.ParticlePairNoExclusions)
         else:
+            force.addEnergyTerm("-138.935485*(1/soluteDielectric-1/solventDielectric)*q1*q2/f;"
+                                "f=sqrt(r^2+B1*B2*exp(-r^2/(4*B1*B2)))", CustomGBForce.ParticlePairNoExclusions)
+    else:
+        if kappa > 0:
             force.addEnergyTerm("-138.935485*(1/soluteDielectric-1/solventDielectric)*q1*q2*(1/f-"+str(1/cutoff)+");"
                                 "f=sqrt(r^2+B1*B2*exp(-r^2/(4*B1*B2)))", CustomGBForce.ParticlePairNoExclusions)
+        else:
+            force.addEnergyTerm("-138.935485*(1/soluteDielectric-kappa/solventDielectric)*q1*q2*(1/f-"+str(1/cutoff)+");"
+                                "f=sqrt(r^2+B1*B2*exp(-r^2/(4*B1*B2)))", CustomGBForce.ParticlePairNoExclusions)
 
 """
 Amber Equivalent: igb = 1