diff --git a/src/unitvector.hh b/src/unitvector.hh
index 446a6a307197e9d2a83c6fb1e166643d01f2f078..f433d64a5750bdfc2af534b4106dfb682934ff1a 100644
--- a/src/unitvector.hh
+++ b/src/unitvector.hh
@@ -115,7 +115,7 @@ public:
         // Bring it all together
         result = A;
         result *= -2;
-        result.axpy(-2*std::acos(sp)/std::sqrt(1-sp*sp), B);
+        result.axpy(derivativeOfArcCosSquared(sp), B);
 
         return result;
     }