diff --git a/triqs/mpi/base.hpp b/triqs/mpi/base.hpp
index 60b54b6e..f0736742 100644
--- a/triqs/mpi/base.hpp
+++ b/triqs/mpi/base.hpp
@@ -19,55 +19,11 @@
  *
  ******************************************************************************/
 #pragma once
-#include <triqs/utility/c14.hpp>
-#include <mpi.h>
-
-namespace boost { // forward declare in case we do not include boost.
-namespace mpi {
- class communicator;
-}
-}
+#include "./communicator.hpp"
 
 namespace triqs {
 namespace mpi {
 
- /// Environment
- struct environment {
-  // MPICH does not allow Init without argc, argv, so we do not allow default constructors
-  // for portability, cf #133
-  environment(int argc, char *argv[]) { MPI_Init(&argc, &argv); }
-  ~environment() { MPI_Finalize(); }
- };
-
- /// The communicator. Todo : add more constructors.
- class communicator {
-  MPI_Comm _com = MPI_COMM_WORLD;
-
-  public:
-  communicator() = default;
-
-  MPI_Comm get() const { return _com; }
-
-  inline communicator(boost::mpi::communicator);
-
-  /// Cast to the boost mpi communicator
-  inline operator boost::mpi::communicator () const;
-
-  int rank() const {
-   int num;
-   MPI_Comm_rank(_com, &num);
-   return num;
-  }
-
-  int size() const {
-   int num;
-   MPI_Comm_size(_com, &num);
-   return num;
-  }
-
-  void barrier() const { MPI_Barrier(_com); }
- };
-
  /// a tag for each operation
  namespace tag {
   struct broadcast {};
diff --git a/triqs/mpi/communicator.hpp b/triqs/mpi/communicator.hpp
new file mode 100644
index 00000000..6afad903
--- /dev/null
+++ b/triqs/mpi/communicator.hpp
@@ -0,0 +1,78 @@
+/*******************************************************************************
+ *
+ * TRIQS: a Toolbox for Research in Interacting Quantum Systems
+ *
+ * Copyright (C) 2014 by O. Parcollet
+ *
+ * TRIQS is free software: you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later
+ * version.
+ *
+ * TRIQS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * TRIQS. If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#pragma once
+#include <triqs/utility/c14.hpp>
+#include <mpi.h>
+
+namespace boost { // forward declare in case we do not include boost.
+namespace mpi {
+ class communicator;
+}
+}
+
+namespace triqs {
+namespace mpi {
+
+ /// Environment
+ struct environment {
+  // MPICH does not allow Init without argc, argv, so we do not allow default constructors
+  // for portability, cf #133
+  environment(int argc, char *argv[]) { MPI_Init(&argc, &argv); }
+  ~environment() { MPI_Finalize(); }
+ };
+
+ // 
+ inline bool is_initialized() noexcept { 
+  int flag;
+  MPI_Initialized(&flag);
+  return flag;
+ }
+
+ /// The communicator. Todo : add more constructors.
+ class communicator {
+  MPI_Comm _com = MPI_COMM_WORLD;
+
+  public:
+  communicator() = default;
+
+  MPI_Comm get() const { return _com; }
+
+  inline communicator(boost::mpi::communicator);
+
+  /// Cast to the boost mpi communicator
+  inline operator boost::mpi::communicator() const;
+
+  int rank() const {
+   int num;
+   MPI_Comm_rank(_com, &num);
+   return num;
+  }
+
+  int size() const {
+   int num;
+   MPI_Comm_size(_com, &num);
+   return num;
+  }
+
+  void barrier() const { MPI_Barrier(_com); }
+ };
+}
+}
diff --git a/triqs/utility/exceptions.hpp b/triqs/utility/exceptions.hpp
index 4c5f979a..2e0014b0 100644
--- a/triqs/utility/exceptions.hpp
+++ b/triqs/utility/exceptions.hpp
@@ -19,10 +19,9 @@
  * TRIQS. If not, see <http://www.gnu.org/licenses/>.
  *
  ******************************************************************************/
+#pragma once
 
-#ifndef TRIQS_EXCEPTIONS_H 
-#define TRIQS_EXCEPTIONS_H
-
+#include "../mpi/communicator.hpp"
 #include "./stack_trace.hpp"
 #include <exception>
 #include <string>
@@ -37,7 +36,13 @@ namespace triqs {
   virtual ~exception() throw() {}
   template<typename T> exception & operator  <<( T const & x) { std::stringstream f; f<<acc<<x; acc = f.str(); return *this;}
   exception & operator  <<( const char * mess ) { (*this) << std::string(mess); return *this;}// to limit code size
-  virtual const char* what() const throw() { return acc.c_str();}
+  virtual const char* what() const throw() {
+   std::stringstream out;
+   out << acc << "\n Error occurred on node ";
+   if (mpi::is_initialized()) out << mpi::communicator().rank()<<"\n"; 
+   out << " C++ trace is : " << trace();
+   return out.str().c_str();
+  }
   virtual const char* trace() const throw() { return _trace.c_str();}
  };
  
@@ -60,5 +65,4 @@ namespace triqs {
 #define TRIQS_RUNTIME_ERROR TRIQS_ERROR(triqs::runtime_error,"runtime error")
 #define TRIQS_KEYBOARD_INTERRUPT TRIQS_ERROR(triqs::keyboard_interrupt,"Ctrl-C")
 
-#endif