Switched back to using exact erf() for exclusions

platforms/cpu/src/CpuNonbondedForce.cpp

@@ -367,15 +367,15 @@ void CpuNonbondedForce::threadComputeDirect(ThreadPool& threads, int threadIndex
                     float inverseR = 1/r;
                     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 erfAlphaR = erf(alphaR);
+                    if (erfAlphaR > 1e-6f) {
                         float dEdR = (float) (chargeProd * inverseR * inverseR * inverseR);
-                        dEdR = (float) (dEdR * (1.0f-erfcAlphaR-TWO_OVER_SQRT_PI*alphaR*exp(-alphaR*alphaR)));
+                        dEdR = (float) (dEdR * (erfAlphaR-TWO_OVER_SQRT_PI*alphaR*exp(-alphaR*alphaR)));
                         fvec4 result = deltaR*dEdR;
                         (fvec4(forces+4*i)-result).store(forces+4*i);
                         (fvec4(forces+4*j)+result).store(forces+4*j);
                         if (includeEnergy)
-                            threadEnergy[threadIndex] -= chargeProd*inverseR*(1.0f-erfcAlphaR);
+                            threadEnergy[threadIndex] -= chargeProd*inverseR*erfAlphaR;
                     }
                 }
             }