diff --git a/src/musrPrimaryGeneratorAction.cc b/src/musrPrimaryGeneratorAction.cc
index 6d79c57..a144e6a 100644
--- a/src/musrPrimaryGeneratorAction.cc
+++ b/src/musrPrimaryGeneratorAction.cc
@@ -274,6 +274,14 @@ void musrPrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
   // works for beam along +-z only for now
   pz = zDirection * std::sqrt(p*p - px*px - py*py);
 
+  // Proper rotation of beam direction as follows:
+  // hat{n} is beam direction unit vecrot and vec{p} is momentum vector
+  // new momentum vector is vec{p'}=R vec{p}
+  // where R is a rotation matrix around (hat{z} x hat{n}) - cross product
+  // with an angle of acos(hat{z} dot hat{n}) - dot product
+  // i.e. R is a rotation around (-n_y, n_x, 0) with angle acos(n_z). 
+  // G4RotationMatrix* R = new G4RotationMatrix(G4ThreeVector(-nx,ny,0),acos(nz)*deg);
+
   // Assign spin 
   G4double xpolaris=0, ypolaris=0, zpolaris=0;
   if (UnpolarisedMuonBeam) {