Reciprocal space force group defaults to "same as direct space force group"

openmmapi/include/openmm/NonbondedForce.h

@@ -267,16 +267,17 @@ public:
      * Get the force group that reciprocal space interactions for Ewald or PME are included in.  This allows multiple
      * time step integrators to evaluate direct and reciprocal space interactions at different intervals: getForceGroup()
      * specifies the group for direct space, and getReciprocalSpaceForceGroup() specifies the group for reciprocal space.
-     * The default value is 0.
+     * If this is -1 (the default value), the same force group is used for reciprocal space as for direct space.
      */
     int getReciprocalSpaceForceGroup() const;
     /**
      * Set the force group that reciprocal space interactions for Ewald or PME are included in.  This allows multiple
      * time step integrators to evaluate direct and reciprocal space interactions at different intervals: setForceGroup()
      * specifies the group for direct space, and setReciprocalSpaceForceGroup() specifies the group for reciprocal space.
-     * The default value is 0.
+     * If this is -1 (the default value), the same force group is used for reciprocal space as for direct space.
      * 
-     * @param group    the group index.  Legal values are between 0 and 31 (inclusive).
+     * @param group    the group index.  Legal values are between 0 and 31 (inclusive), or -1 to use the same force group
+     *                 that is specified for direct space.
      */
     void setReciprocalSpaceForceGroup(int group);
 protected:

openmmapi/src/NonbondedForce.cpp

@@ -47,7 +47,7 @@ using std::string;
 using std::stringstream;
 using std::vector;
 
-NonbondedForce::NonbondedForce() : nonbondedMethod(NoCutoff), cutoffDistance(1.0), rfDielectric(78.3), ewaldErrorTol(5e-4), useDispersionCorrection(true), recipForceGroup(0) {
+NonbondedForce::NonbondedForce() : nonbondedMethod(NoCutoff), cutoffDistance(1.0), rfDielectric(78.3), ewaldErrorTol(5e-4), useDispersionCorrection(true), recipForceGroup(-1) {
 }
 
 NonbondedForce::NonbondedMethod NonbondedForce::getNonbondedMethod() const {
@@ -202,7 +202,7 @@ int NonbondedForce::getReciprocalSpaceForceGroup() const {
 }
 
 void NonbondedForce::setReciprocalSpaceForceGroup(int group) {
-    if (group < 0 || group > 31)
-        throw OpenMMException("Force group must be between 0 and 31");
+    if (group < -1 || group > 31)
+        throw OpenMMException("Force group must be between -1 and 31");
     recipForceGroup = group;
 }

openmmapi/src/NonbondedForceImpl.cpp

@@ -100,7 +100,9 @@ void NonbondedForceImpl::initialize(ContextImpl& context) {
 
 double NonbondedForceImpl::calcForcesAndEnergy(ContextImpl& context, bool includeForces, bool includeEnergy, int groups) {
     bool includeDirect = ((groups&(1<<owner.getForceGroup())) != 0);
-    bool includeReciprocal = ((groups&(1<<owner.getReciprocalSpaceForceGroup())) != 0);
+    bool includeReciprocal = includeDirect;
+    if (owner.getReciprocalSpaceForceGroup() >= 0)
+        includeReciprocal = ((groups&(1<<owner.getReciprocalSpaceForceGroup())) != 0);
     return dynamic_cast<CalcNonbondedForceKernel&>(kernel.getImpl()).execute(context, includeForces, includeEnergy, includeDirect, includeReciprocal);
 }
 

platforms/cuda/src/CudaKernels.cpp

@@ -822,7 +822,7 @@ void CudaCalcNonbondedForceKernel::initialize(const System& system, const Nonbon
             method = PERIODIC;
         }
         if (force.getNonbondedMethod() == NonbondedForce::Ewald || force.getNonbondedMethod() == NonbondedForce::PME) {
-            if (force.getReciprocalSpaceForceGroup() != 0)
+            if (force.getReciprocalSpaceForceGroup() > 0)
                 throw OpenMMException("CudaPlatform does not support force groups");
             if (force.getNonbondedMethod() == NonbondedForce::Ewald) {
                 double alpha;