diff --git a/src/Makefile b/src/Makefile
index 2c63895..bb66f90 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -136,6 +136,9 @@ CXXFLAGS += -DTARGET='"$(TARGET)"'
  testFunction3d: testFunction3d.o Function3d.o Function3dHandler.o \
         Base64Transcoder.o qbox_xmlns.o $(PLTOBJECTS)
 	$(LD) $(DFLAGS) -o $@ $^ $(LDFLAGS)
+ test_vext: test_vext.o Function3d.o Function3dHandler.o \
+        Base64Transcoder.o qbox_xmlns.o $(PLTOBJECTS)
+	$(LD) $(DFLAGS) -o $@ $^ $(LDFLAGS)
 #------------------------------------------------------------------------------
 # generate dependencies in makefile: use -Y to avoid library header files
 # that are likely to be different on other platforms.
@@ -579,8 +582,7 @@ ResponseCmd.o: blacs.h D3vector.h Wavefunction.h UnitCell.h SlaterDet.h
 ResponseCmd.o: Basis.h Sample.h AtomSet.h Atom.h D3tensor.h blas.h
 ResponseCmd.o: ConstraintSet.h ExtForceSet.h Control.h ChargeDensity.h
 ResponseCmd.o: ExternalPotential.h FourierTransform.h ResponseCmd.h
-ResponseCmd.o: UserInterface.h release.h isodate.h Species.h
-ResponseCmd.o: Base64Transcoder.h qbox_xmlns.h
+ResponseCmd.o: UserInterface.h release.h isodate.h Species.h Function3d.h
 ResponseCmd.o: UserInterface.h
 RseedCmd.o: UserInterface.h Sample.h AtomSet.h Context.h blacs.h Atom.h
 RseedCmd.o: D3vector.h UnitCell.h D3tensor.h blas.h ConstraintSet.h
@@ -820,6 +822,7 @@ testWavefunction.o: SlaterDet.h Basis.h Matrix.h Timer.h
 testXCFunctional.o: LDAFunctional.h XCFunctional.h PBEFunctional.h Timer.h
 testXMLGFPreprocessor.o: Context.h blacs.h Matrix.h XMLGFPreprocessor.h
 test_fftw.o: Timer.h readTSC.h
+test_vext.o: Function3d.h D3vector.h
 testjacobi.o: Timer.h Context.h blacs.h Matrix.h jacobi.h
 testjade.o: Timer.h Context.h blacs.h Matrix.h jade.h
 uuid_str.o: uuid_str.h
diff --git a/src/test_vext.C b/src/test_vext.C
new file mode 100644
index 0000000..32559b6
--- /dev/null
+++ b/src/test_vext.C
@@ -0,0 +1,61 @@
+////////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2018 The Regents of the University of California
+//
+// This file is part of Qbox
+//
+// Qbox is distributed 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.
+// See the file COPYING in the root directory of this distribution
+// or <http://www.gnu.org/licenses/>.
+//
+////////////////////////////////////////////////////////////////////////////////
+//
+// test_vext.C:
+//
+// Generate an external potential in XML format for input to the response
+// command.
+// vext = sine wave in the x direction
+// use: test_vext a b c np0 np1 np2
+//
+////////////////////////////////////////////////////////////////////////////////
+
+#include "Function3d.h"
+#include<cassert>
+#include<vector>
+#include<string>
+using namespace std;
+
+////////////////////////////////////////////////////////////////////////////////
+int main(int argc, char **argv)
+{
+  if ( argc == 1 )
+  {
+    cerr << "use: test_vext a b c np0 np1 np2" << endl;
+    return 1;
+  }
+  double a = atof(argv[1]);
+  double b = atof(argv[2]);
+  double c = atof(argv[3]);
+  Function3d f;
+  f.a = D3vector(a,0,0);
+  f.b = D3vector(0,b,0);
+  f.c = D3vector(0,0,c);
+  f.nx = atoi(argv[4]);
+  f.ny = atoi(argv[5]);
+  f.nz = atoi(argv[6]);
+  f.val.resize(f.nx*f.ny*f.nz);
+  f.name = "delta_v";
+  for ( int i = 0; i < f.nx; i++ )
+    for ( int j = 0; j < f.ny; j++ )
+      for ( int k = 0; k < f.nz; k++ )
+      {
+        double x = ( a * i ) / f.nx;
+        double y = ( b * j ) / f.ny;
+        double z = ( c * k ) / f.nz;
+        f.val[i+f.nx*(j+f.ny*k)] = sin(2*M_PI*x/a);
+      }
+  cout << f;
+  return 0;
+}