diff --git a/src/Utils/PhysConst.h b/src/Utils/PhysConst.h
index 6d89108..c71377d 100644
--- a/src/Utils/PhysConst.h
+++ b/src/Utils/PhysConst.h
@@ -1,76 +1,77 @@
 // Copyright 2016 L. Pickering, P Stowell, R. Terri, C. Wilkinson, C. Wret
 
 /*******************************************************************************
 *    This file is part of NUISANCE.
 *
 *    NUISANCE 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.
 *
 *    NUISANCE 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 NUISANCE.  If not, see <http://www.gnu.org/licenses/>.
 *******************************************************************************/
 #ifndef PHYSCONST_H_SEEN
 #define PHYSCONST_H_SEEN
 
 #include <math.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <cstring>
 #include <fstream>
 #include <iostream>
 #include <iostream>
 #include <limits>
 #include <numeric>
 #include <sstream>
 #include <string>
 #include <vector>
 #include "FitLogger.h"
 
 /// namespace to contain all physical constants used by NUISANCE
 namespace PhysConst {
 const double mass_proton = 0.93827203;        // Proton mass in GeV
 const double mass_neutron = 0.93956536;       // Neutron mass in GeV
+const double mass_nucleon = (mass_proton + mass_neutron)/2.;
 const double mass_delta = 1.232;              // Delta mass in GeV
 const double mass_muon = 0.10565837;          // Muon mass in GeV
 const double mass_electron = 0.000510998928;  // Electron mass in GeV
 const double mass_cpi = 0.13957;              // charged pion mass in GeV
 const double mass_pi0 = 0.13498;              // neutral pion mass in GeV
 
 const double mass_MeV = 1000;  // MeV/GeV
 
 const int pdg_neutrinos[] = {12, -12, 14, -14 /*, 16, -16*/};
 const int pdg_muons[] = {13, -13};
 const int pdg_leptons[] = {11, -11, 13, -13, 15, -15};
 const int pdg_all_leptons[] = {11, -11, 13, -13, 15, -15,
                                12, -12, 14, -14, 16, -16};
 
 const int pdg_pions[] = {211, -211, 111};
 const int pdg_charged_pions[] = {211, -211};
 const int pdg_strangemesons[] = {
     130,     310,     311,     321,     9000311, 9000321, 10311,
     10321,   100311,  100321,  9010311, 9010321, 9020311, 9020321,
     313,     323,     10313,   10323,   20313,   20323,   100313,
     100323,  9000313, 9000323, 30313,   30323,   315,     325,
     9000315, 9000325, 10315,   10325,   20315,   20325,   9010315,
     9010325, 9020315, 9020325, 317,     327,     9010317, 9010327};
 
 // Just *-1 to cover possibility
 const int pdg_kplus = 321;
 const int pdg_antistrangemesons[] = {
     -130,     -310,     -311,     -321,     -9000311, -9000321, -10311,
     -10321,   -100311,  -100321,  -9010311, -9010321, -9020311, -9020321,
     -313,     -323,     -10313,   -10323,   -20313,   -20323,   -100313,
     -100323,  -9000313, -9000323, -30313,   -30323,   -315,     -325,
     -9000315, -9000325, -10315,   -10325,   -20315,   -20325,   -9010315,
     -9010325, -9020315, -9020325, -317,     -327,     -9010317, -9010327};
 }
 
 /*! @} */
 #endif