diff --git a/Geo/STensor3.h b/Geo/STensor3.h
index 220fde14dfa4a636cd1353e3f61a1a5f11fc1a1c..11cf2f040d2bfb0a8c3d96e00654da735aa7cb30 100644
--- a/Geo/STensor3.h
+++ b/Geo/STensor3.h
@@ -350,6 +350,14 @@ class STensor3 {
     }
     return sqrt(sqr);
   }
+  STensor3 dev() const{
+    double p= trace()/3.;
+    STensor3 de(*this);
+    de(0,0) -= p;
+    de(1,1) -= p;
+    de(2,2) -= p;
+    return de;
+  }
 };
 
 // tensor product