Tolerate round-off error when checking the box size against the cutoff distance

platforms/cuda/src/CudaKernels.cpp

@@ -52,7 +52,7 @@ void CudaCalcForcesAndEnergyKernel::beginComputation(ContextImpl& context, bool
         gpuReorderAtoms(gpu);
     if ((data.hasNonbonded && data.nonbondedMethod != NO_CUTOFF && data.nonbondedMethod != CUTOFF) ||
         (data.hasCustomNonbonded && data.customNonbondedMethod != NO_CUTOFF && data.customNonbondedMethod != CUTOFF)) {
-        double minAllowedSize = 2*gpu->sim.nonbondedCutoff;
+        double minAllowedSize = 1.999999*gpu->sim.nonbondedCutoff;
         if (gpu->sim.periodicBoxSizeX < minAllowedSize || gpu->sim.periodicBoxSizeY < minAllowedSize || gpu->sim.periodicBoxSizeZ < minAllowedSize)
             throw OpenMMException("The periodic box size has decreased to less than twice the nonbonded cutoff.");
     }

platforms/opencl/src/OpenCLNonbondedUtilities.cpp

@@ -320,7 +320,7 @@ void OpenCLNonbondedUtilities::prepareInteractions() {
         return;
     if (usePeriodic) {
         mm_float4 box = context.getPeriodicBoxSize();
-        double minAllowedSize = 2*cutoff;
+        double minAllowedSize = 1.999999*cutoff;
         if (box.x < minAllowedSize || box.y < minAllowedSize || box.z < minAllowedSize)
             throw OpenMMException("The periodic box size has decreased to less than twice the nonbonded cutoff.");
     }

platforms/reference/src/ReferenceKernels.cpp

@@ -690,7 +690,7 @@ double ReferenceCalcNonbondedForceKernel::execute(ContextImpl& context, bool inc
     }
     if (periodic || ewald || pme) {
         RealVec& box = extractBoxSize(context);
-        double minAllowedSize = 2*nonbondedCutoff;
+        double minAllowedSize = 1.999999*nonbondedCutoff;
         if (box[0] < minAllowedSize || box[1] < minAllowedSize || box[2] < minAllowedSize)
             throw OpenMMException("The periodic box size has decreased to less than twice the nonbonded cutoff.");
         clj.setPeriodic(box);