Fixed error in accumulating parameter derivatives (#4249)

platforms/common/src/CommonKernels.cpp

@@ -2315,7 +2315,7 @@ void CommonCalcCustomNonbondedForceKernel::initInteractionGroups(const CustomNon
         initDerivs<<"mixed "<<derivVariable<<" = 0;\n";
         for (int index = 0; index < numDerivs; index++)
             if (allParamDerivNames[index] == paramName)
-                saveDerivs<<"energyParamDerivs[GLOBAL_ID*"<<numDerivs<<"+"<<index<<"] += "<<derivVariable<<";\n";
+                saveDerivs<<"energyParamDerivs[GLOBAL_ID*numDerivatives+"<<index<<"] += "<<derivVariable<<";\n";
     }
     replacements["INIT_DERIVATIVES"] = initDerivs.str();
     replacements["SAVE_DERIVATIVES"] = saveDerivs.str();
@@ -2391,6 +2391,7 @@ double CommonCalcCustomNonbondedForceKernel::execute(ContextImpl& context, bool
             interactionGroupKernel->addArg((int) useNeighborList);
             for (int i = 0; i < 5; i++)
                 interactionGroupKernel->addArg(); // Periodic box information will be set just before it is executed.
+            interactionGroupKernel->addArg((int) cc.getEnergyParamDerivNames().size());
             for (auto& buffer : paramBuffers)
                 interactionGroupKernel->addArg(buffer.getArray());
             for (auto& buffer : computedValueBuffers)

platforms/common/src/kernels/customNonbondedGroups.cc

@@ -39,7 +39,7 @@ KERNEL void computeInteractionGroups(
         GLOBAL mm_ulong* RESTRICT forceBuffers,
         GLOBAL mixed* RESTRICT energyBuffer, GLOBAL const real4* RESTRICT posq, GLOBAL const int4* RESTRICT groupData,
         GLOBAL const int* RESTRICT numGroupTiles, int useNeighborList,
-        real4 periodicBoxSize, real4 invPeriodicBoxSize, real4 periodicBoxVecX, real4 periodicBoxVecY, real4 periodicBoxVecZ
+        real4 periodicBoxSize, real4 invPeriodicBoxSize, real4 periodicBoxVecX, real4 periodicBoxVecY, real4 periodicBoxVecZ, int numDerivatives
         PARAMETER_ARGUMENTS) {
     const unsigned int totalWarps = GLOBAL_SIZE/TILE_SIZE;
     const unsigned int warp = GLOBAL_ID/TILE_SIZE; // global warpIndex