Illustrates the calulations needed to generate the interatomic force constants file
by using the finite displacements method

1) make a self-consistent calculation 
   cd $espresso_dir/PHonon/FD/example/inputfiles
   $espresso_dir/bin/pw.x < si.scf.in > si.scf.out
  
2) create finite displacements in a supercell
   $espresso_dir/bin/fd.x < fd.in > fd.out

3) run all the finite displacements and extract the forces
   sh ../../run_fd 

4) generate the file "si_ifc.fc" of the interatomic force constants
   $espresso_dir/bin/fd_ifc.x < fd_ifc.in
   cat ./fd_files/header.txt > si_ifc.fc
   echo "F" >> si_ifc.fc
   cat ./silicon_ifc.fc >> si_ifc.fc

