3rd variant of Fortran wrapper for DFTB+ calling LAMMPS

examples/COUPLE/fortran3/README

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+This directory has an example of using a callback function to obtain
+forces from a fortran code for a LAMMPS simulation.  The reader should
+refer to the README file in COUPLE/fortran2 before proceeding. Here,
+the LAMMPS.F90 file has been modified slightly and additional files
+named LAMMPS-wrapper2.h and LAMMPS-wrapper2.cpp have been included in
+order to supply wrapper functions to set the LAMMPS callback function
+and total energy.
+
+In this example, the callback function is set to run the
+semi-empirical quantum code DFTB+ in serial and then read in the total
+energy, forces, and stress tensor from file.  In this case, nlocal =
+the total number of atoms in the system, so particle positions can be
+read from the pos array directly, and DFTB+ forces can simply be
+included via the fext array. The user should take care in the case of
+a parallel calculation, where LAMMPS can assign different particules
+to each processor. For example, the user should use functions such as
+lammps_gather_atoms() and lammps_scatter_atoms() in the case where the
+fortran force calculating code requires the positions of all atoms,
+etc.
+
+A few more important notes: 
+
+-The stress tensor from DFTB+ is passed in to LAMMPS via pointer.
+-Calling the subroutine lammps_set_callback() is required in order to set
+       a pointer to the callback function in LAMMPS.
+-The subroutine lammps_set_user_energy() passes in the potential energy
+       from DFTB+ to LAMMPS.
+
+This example was created by Nir Goldman, whom you can contact with
+questions:
+
+Nir Goldman, LLNL
+ngoldman@llnl.gov