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, total energy, virial, and electronic entropy contribution (needed for MSST simulations with a quantum code).

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:

-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. Similarly, lammps_set_user_virial passes the stress tensor.

-The electronic entropy contribution is set via lammps_set_external_vector(). Their needs to be a call to lammps_set_external_vector_length() before this value can be passed to LAMMPS.

This example was created by Nir Goldman, whom you can contact with questions:

Nir Goldman, LLNL ngoldman@llnl.gov