diff --git a/tests/forces/Test_zmobius_force.cc b/tests/forces/Test_zmobius_force.cc
new file mode 100644
index 00000000..d3a521c1
--- /dev/null
+++ b/tests/forces/Test_zmobius_force.cc
@@ -0,0 +1,167 @@
+    /*************************************************************************************
+
+    Grid physics library, www.github.com/paboyle/Grid 
+
+    Source file: ./tests/Test_dwf_force.cc
+
+    Copyright (C) 2015
+
+Author: Peter Boyle <paboyle@ph.ed.ac.uk>
+
+    This program 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 2 of the License, or
+    (at your option) any later version.
+
+    This program 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 this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+    See the full license in the file "LICENSE" in the top level distribution directory
+    *************************************************************************************/
+    /*  END LEGAL */
+#include <Grid/Grid.h>
+
+using namespace std;
+using namespace Grid;
+using namespace Grid::QCD;
+
+int main (int argc, char ** argv)
+{
+  Grid_init(&argc,&argv);
+
+  std::vector<int> latt_size   = GridDefaultLatt();
+  std::vector<int> simd_layout = GridDefaultSimd(Nd,vComplex::Nsimd());
+  std::vector<int> mpi_layout  = GridDefaultMpi();
+
+  const int Ls=8;
+
+  GridCartesian         * UGrid   = SpaceTimeGrid::makeFourDimGrid(GridDefaultLatt(), GridDefaultSimd(Nd,vComplex::Nsimd()),GridDefaultMpi());
+  GridRedBlackCartesian * UrbGrid = SpaceTimeGrid::makeFourDimRedBlackGrid(UGrid);
+  GridCartesian         * FGrid   = SpaceTimeGrid::makeFiveDimGrid(Ls,UGrid);
+  GridRedBlackCartesian * FrbGrid = SpaceTimeGrid::makeFiveDimRedBlackGrid(Ls,UGrid);
+
+  std::vector<int> seeds4({1,2,3,4});
+  std::vector<int> seeds5({5,6,7,8});
+  GridParallelRNG          RNG5(FGrid);  RNG5.SeedFixedIntegers(seeds5);
+  GridParallelRNG          RNG4(UGrid);  RNG4.SeedFixedIntegers(seeds4);
+
+  int threads = GridThread::GetThreads();
+  std::cout<<GridLogMessage << "Grid is setup to use "<<threads<<" threads"<<std::endl;
+
+  LatticeFermion phi        (FGrid); gaussian(RNG5,phi);
+  LatticeFermion Mphi       (FGrid); 
+  LatticeFermion MphiPrime  (FGrid); 
+
+  LatticeGaugeField U(UGrid);
+
+  SU3::HotConfiguration(RNG4,U);
+  
+  ////////////////////////////////////
+  // Unmodified matrix element
+  ////////////////////////////////////
+  RealD mass=0.01; 
+  RealD M5=1.8; 
+  RealD b=0.5;
+  RealD c=0.5;
+
+  std::vector < std::complex<double>  > omegas;
+  omegas.push_back( std::complex<double>(1.45806438985048,-0) );
+  omegas.push_back( std::complex<double>(1.18231318389348,-0) );
+  omegas.push_back( std::complex<double>(0.830951166685955,-0) );
+  omegas.push_back( std::complex<double>(0.542352409156791,-0) );
+  omegas.push_back( std::complex<double>(0.341985020453729,-0) );
+  omegas.push_back( std::complex<double>(0.21137902619029,-0) );
+  omegas.push_back( std::complex<double>(0.126074299502912,-0) );
+  omegas.push_back( std::complex<double>(0.0990136651962626,-0) );
+  omegas.push_back( std::complex<double>(0.0686324988446592,0.0550658530827402) );
+  omegas.push_back( std::complex<double>(0.0686324988446592,-0.0550658530827402) );
+
+  ZMobiusFermionR Ddwf(U, *FGrid, *FrbGrid, *UGrid, *UrbGrid, mass, M5, omegas,b,c);
+
+  Ddwf.M   (phi,Mphi);
+
+  ComplexD S    = innerProduct(Mphi,Mphi); // pdag MdagM p
+
+  // get the deriv of phidag MdagM phi with respect to "U"
+  LatticeGaugeField UdSdU(UGrid);
+  LatticeGaugeField tmp(UGrid);
+  
+
+  Ddwf.MDeriv(tmp , Mphi,  phi,DaggerNo );  UdSdU=tmp;
+  Ddwf.MDeriv(tmp , phi,  Mphi,DaggerYes ); UdSdU=(UdSdU+tmp);  
+  
+  LatticeFermion Ftmp      (FGrid);
+
+  ////////////////////////////////////
+  // Modify the gauge field a little 
+  ////////////////////////////////////
+  RealD dt = 0.0001;
+
+  LatticeColourMatrix mommu(UGrid); 
+  LatticeColourMatrix forcemu(UGrid); 
+  LatticeGaugeField mom(UGrid); 
+  LatticeGaugeField Uprime(UGrid); 
+
+  for(int mu=0;mu<Nd;mu++){
+
+    SU3::GaussianFundamentalLieAlgebraMatrix(RNG4, mommu); // Traceless antihermitian momentum; gaussian in lie alg
+
+    PokeIndex<LorentzIndex>(mom,mommu,mu);
+
+    // fourth order exponential approx
+    parallel_for(auto i=mom.begin();i<mom.end();i++){
+      Uprime[i](mu) =
+	  U[i](mu)
+	+ mom[i](mu)*U[i](mu)*dt 
+	+ mom[i](mu) *mom[i](mu) *U[i](mu)*(dt*dt/2.0)
+	+ mom[i](mu) *mom[i](mu) *mom[i](mu) *U[i](mu)*(dt*dt*dt/6.0)
+	+ mom[i](mu) *mom[i](mu) *mom[i](mu) *mom[i](mu) *U[i](mu)*(dt*dt*dt*dt/24.0)
+	+ mom[i](mu) *mom[i](mu) *mom[i](mu) *mom[i](mu) *mom[i](mu) *U[i](mu)*(dt*dt*dt*dt*dt/120.0)
+	+ mom[i](mu) *mom[i](mu) *mom[i](mu) *mom[i](mu) *mom[i](mu) *mom[i](mu) *U[i](mu)*(dt*dt*dt*dt*dt*dt/720.0)
+	;
+    }
+
+  }
+  
+  Ddwf.ImportGauge(Uprime);
+  Ddwf.M          (phi,MphiPrime);
+
+  ComplexD Sprime    = innerProduct(MphiPrime   ,MphiPrime);
+
+  //////////////////////////////////////////////
+  // Use derivative to estimate dS
+  //////////////////////////////////////////////
+
+
+  LatticeComplex dS(UGrid); dS = zero;
+  for(int mu=0;mu<Nd;mu++){
+    mommu   = PeekIndex<LorentzIndex>(UdSdU,mu);
+    mommu=Ta(mommu)*2.0;
+    PokeIndex<LorentzIndex>(UdSdU,mommu,mu);
+  }
+
+  for(int mu=0;mu<Nd;mu++){
+    forcemu = PeekIndex<LorentzIndex>(UdSdU,mu);
+    mommu   = PeekIndex<LorentzIndex>(mom,mu);
+
+    // Update PF action density
+    dS = dS+trace(mommu*forcemu)*dt;
+
+  }
+
+  Complex dSpred    = sum(dS);
+
+  std::cout << GridLogMessage << " S      "<<S<<std::endl;
+  std::cout << GridLogMessage << " Sprime "<<Sprime<<std::endl;
+  std::cout << GridLogMessage << "dS      "<<Sprime-S<<std::endl;
+  std::cout << GridLogMessage << "predict dS    "<< dSpred <<std::endl;
+
+  std::cout<< GridLogMessage << "Done" <<std::endl;
+  Grid_finalize();
+}