Removed more unnecessarily restrictive parameter checks (#3326)

openmmapi/src/HarmonicAngleForceImpl.cpp

@@ -64,8 +64,6 @@ void HarmonicAngleForceImpl::initialize(ContextImpl& context) {
         }
         if (angle < 0 || angle > M_PI*1.000001)
             throw OpenMMException("HarmonicAngleForce: angle must be between 0 and pi");
-        if (k < 0)
-            throw OpenMMException("HarmonicAngleForce: force constant cannot be negative");
     }
     kernel = context.getPlatform().createKernel(CalcHarmonicAngleForceKernel::Name(), context);
     kernel.getAs<CalcHarmonicAngleForceKernel>().initialize(context.getSystem(), owner);

openmmapi/src/NonbondedForceImpl.cpp

@@ -93,8 +93,8 @@ void NonbondedForceImpl::initialize(ContextImpl& context) {
         }
         exceptions[particle[0]].insert(particle[1]);
         exceptions[particle[1]].insert(particle[0]);
-        if (sigma <= 0)
-            throw OpenMMException("NonbondedForce: sigma for an exception must be positive");
+        if (sigma < 0)
+            throw OpenMMException("NonbondedForce: sigma for an exception cannot be negative");
         if (epsilon < 0)
             throw OpenMMException("NonbondedForce: epsilon for an exception cannot be negative");
     }
@@ -121,10 +121,6 @@ void NonbondedForceImpl::initialize(ContextImpl& context) {
             msg << exceptionIndex;
             throw OpenMMException(msg.str());
         }
-        if (sigmaScale < 0)
-            throw OpenMMException("NonbondedForce: sigma scale for a particle parameter offset cannot be negative");
-        if (epsilonScale < 0)
-            throw OpenMMException("NonbondedForce: epsilon scale for a particle parameter offset cannot be negative");
     }
     if (owner.getNonbondedMethod() != NonbondedForce::NoCutoff && owner.getNonbondedMethod() != NonbondedForce::CutoffNonPeriodic) {
         Vec3 boxVectors[3];