Prevent nan in CPU platform when two excluded particles are exactly on top of each other

platforms/cpu/src/CpuNonbondedForce.cpp

@@ -360,6 +360,7 @@ void CpuNonbondedForce::threadComputeDirect(ThreadPool& threads, int threadIndex
                     float chargeProd = ONE_4PI_EPS0*posq[4*i+3]*posq[4*j+3];
                     float alphaR = alphaEwald*r;
                     float erfcAlphaR = erfcApprox(alphaR);
+                    if (1-erfcAlphaR > 1e-6f) {
                         float dEdR = (float) (chargeProd * inverseR * inverseR * inverseR);
                         dEdR = (float) (dEdR * (1.0f-erfcAlphaR-TWO_OVER_SQRT_PI*alphaR*exp(-alphaR*alphaR)));
                         fvec4 result = deltaR*dEdR;
@@ -371,6 +372,7 @@ void CpuNonbondedForce::threadComputeDirect(ThreadPool& threads, int threadIndex
                 }
             }
         }
+    }
     else if (cutoff) {
         // Compute the interactions from the neighbor list.