diff --git a/include/SimCore/Generators/GenieGenerator.h b/include/SimCore/Generators/GenieGenerator.h
index 02a8132..8bd928c 100644
--- a/include/SimCore/Generators/GenieGenerator.h
+++ b/include/SimCore/Generators/GenieGenerator.h
@@ -80,6 +80,7 @@ class GenieGenerator : public simcore::PrimaryGenerator {
   std::vector<int> targets_;
   std::vector<double> abundances_;
   std::vector<double> position_;
+  std::vector<double> beam_size_;
   double target_thickness_;
   double time_;
   std::vector<double> direction_;
diff --git a/python/generators.py.in b/python/generators.py.in
index 8e55197..6948e9b 100644
--- a/python/generators.py.in
+++ b/python/generators.py.in
@@ -219,6 +219,10 @@ class genie(simcfg.PrimaryGenerator) :
         Unit vector direction of incoming electron
     tune: str
         Name of GENIE tune to use
+    target_thickness : double
+        Thickness of target to use for generation
+    beam_size : list of double
+        uniform beam size width to use
 
     Examples
     --------
@@ -240,6 +244,7 @@ class genie(simcfg.PrimaryGenerator) :
                      abundances = [],
                      time = 0.0,
                      position = [ 0.0, 0.0, 0.0 ],
+                     beam_size = [ 0.0, 0.0 ],
                      direction = [ 0.0, 0.0, 1.0 ],
                      tune = 'default',
                      spline_file = '',
@@ -253,6 +258,7 @@ class genie(simcfg.PrimaryGenerator) :
         self.abundances = abundances
         self.time = time
         self.position = position
+        self.beam_size = beam_size
         self.direction = direction
         self.tune = tune
         self.spline_file = spline_file
diff --git a/src/SimCore/Generators/GenieGenerator.cxx b/src/SimCore/Generators/GenieGenerator.cxx
index 7ffcc6d..2b85cb8 100644
--- a/src/SimCore/Generators/GenieGenerator.cxx
+++ b/src/SimCore/Generators/GenieGenerator.cxx
@@ -72,6 +72,7 @@ void GenieGenerator::fillConfig(const framework::config::Parameters& p)
 
   time_        = p.getParameter<double>("time");// * ns;
   position_    = p.getParameter< std::vector<double> >("position"); // mm
+  beam_size_   = p.getParameter< std::vector<double> >("beam_size"); // mm
   direction_   = p.getParameter< std::vector<double> >("direction");
   target_thickness_ = p.getParameter<double>("target_thickness"); //mm
 
@@ -119,6 +120,30 @@ bool GenieGenerator::validateConfig()
       target_thickness_ = std::abs(target_thickness_);
     }
 
+  if(beam_size_.size()!=2)
+    {
+      if (beam_size_.size()==0){
+	std::cout << "beam size not set. Using zero."
+		  << std::endl;
+	beam_size_.resize(2); beam_size_[0] = 0.0; beam_size_[1] = 0.0;
+      }
+      else{
+	std::cout << "beam size is set, but does not have size 2."
+		  << " " << beam_size_.size()
+		  << std::endl;
+	ret=false;
+      }
+    }
+  else if(beam_size_[0] < 0 || beam_size_[1] < 0 )
+    {
+      std::cout << "Beam size set as negative value? "
+		<< "(" << beam_size_[0] << "," << beam_size_[1] << ")"
+		<< std::endl;
+      std::cout << "Changing to positive." << std::endl;
+      beam_size_[0] = std::abs(beam_size_[0]);
+      beam_size_[1] = std::abs(beam_size_[1]);      
+    }
+
   //normalize abundances
   double abundance_sum=0;
   for( auto a : abundances_){
@@ -319,10 +344,12 @@ void GenieGenerator::GeneratePrimaryVertex(G4Event* event)
       
     }
 
+  auto x_pos = position_[0] + (G4Random::getTheGenerator()->flat()-0.5)*beam_size_[0];
+  auto y_pos = position_[1] + (G4Random::getTheGenerator()->flat()-0.5)*beam_size_[1];
   auto z_pos = position_[2] + (G4Random::getTheGenerator()->flat()-0.5)*target_thickness_;
   if(verbosity_>=1)
     std::cout << "Generating interaction at (x,y,z)="
-	      << "(" << position_[0] << "," << position_[1] << "," << z_pos << ")"
+	      << "(" << x_pos << "," << y_pos << "," << z_pos << ")"
 	      << std::endl;
   
   genie::InitialState initial_state(targets_.at(nucl_target_i),11);
@@ -332,13 +359,17 @@ void GenieGenerator::GeneratePrimaryVertex(G4Event* event)
   //setup the initial election
   TParticle initial_e;
   initial_e.SetPdgCode(11);
-  auto elec_i_p = std::sqrt(energy_*energy_ - initial_e.GetMass()*initial_e.GetMass());
+  double elec_i_p = std::sqrt(energy_*energy_ - (double)initial_e.GetMass()*(double)initial_e.GetMass());
   initial_e.SetMomentum(elec_i_p*direction_[0],
 			elec_i_p*direction_[1],
 			elec_i_p*direction_[2],
 			energy_);
   TLorentzVector e_p4;
   initial_e.Momentum(e_p4);
+  if(verbosity_>=1)
+    std::cout << "Generating interation with (px,py,pz,e)="
+	      << "(" << e_p4.Px() << "," << e_p4.Py() << "," << e_p4.Pz() << "," << e_p4.E() << ")"
+	      << std::endl;
 
   //calculate total xsec
   if( n_events_by_target_[nucl_target_i]==0)
@@ -359,8 +390,8 @@ void GenieGenerator::GeneratePrimaryVertex(G4Event* event)
   //setup the primary vertex now
   
   G4PrimaryVertex* vertex = new G4PrimaryVertex();
-  vertex->SetPosition(position_[0],
-		      position_[1],
+  vertex->SetPosition(x_pos,
+		      y_pos,
 		      z_pos);
   vertex->SetWeight(genie_event->Weight());
   
diff --git a/src/SimCore/Simulator.cxx b/src/SimCore/Simulator.cxx
index 22332f4..bab2183 100644
--- a/src/SimCore/Simulator.cxx
+++ b/src/SimCore/Simulator.cxx
@@ -155,10 +155,10 @@ void Simulator::FillGHepParticles(const genie::EventRecord* record,
       my_particle.fFirstDaugher = particle->FirstDaughter();
       my_particle.fLastDaughter = particle->LastDaughter();
       
-      my_particle.fP_x = particle->P4()->X();
-      my_particle.fP_y = particle->P4()->Y();
-      my_particle.fP_z = particle->P4()->Z();
-      my_particle.fP_t = particle->P4()->T();
+      my_particle.fP_x = particle->P4()->Px();
+      my_particle.fP_y = particle->P4()->Py();
+      my_particle.fP_z = particle->P4()->Pz();
+      my_particle.fP_t = particle->P4()->E();
       
       my_particle.fX_x = particle->X4()->X();
       my_particle.fX_y = particle->X4()->Y();