This directory has a simple C and C++ code that shows how LAMMPS can be linked to a driver application as a library. The purpose is to illustrate how another code could perform computations while using LAMMPS to perform MD, or how an umbrella code or script could call both LAMMPS and some other code to perform a coupled calculation.

simple.cpp is the C++ driver simple.c is the C driver

The 2 codes do the same thing, so you can compare them to see how to drive LAMMPS in this manner. The C driver is similar in spirit to what one could use from a Fortran program or scripting language.

You can then build either driver code with a compile line something like this, which includes paths to the LAMMPS library interface, MPI, and FFTW (assuming you built LAMMPS as a library with its PPPM solver).

This builds the C++ driver with the LAMMPS library using a C++ compiler:

g++ -I/home/sjplimp/lammps/src -c simple.cpp g++ -L/home/sjplimp/lammps/src simple.o \

-llmp_g++ -lfftw -lmpich -lpthread -o simpleCC

This builds the C driver with the LAMMPS library using a C compiler:

gcc -I/home/sjplimp/lammps/src -c simple.c gcc -L/home/sjplimp/lammps/src simple.o \

-llmp_g++ -lfftw -lmpich -lpthread -lstdc++ -o simpleC

You then run simpleCC or simpleC on a parallel machine on some number of processors Q with 2 arguments:

mpirun -np Q simpleCC P in.lj

P is the number of procs you want LAMMPS to run on (must be <= Q) and in.lj is a LAMMPS input script.

The driver will launch LAMMPS on P procs, read the input script a line at a time, and pass each command line to LAMMPS. The final line of the script is a "run" command, so LAMMPS will run the problem.

The driver then requests all the atom coordinates from LAMMPS, moves one of the atoms a small amount "epsilon", passes the coordinates back to LAMMPS, and runs LAMMPS again. If you look at the output, you should see a small energy change between runs, due to the moved atom.

The C driver is calling C-style routines in the src/library.cpp file of LAMMPS. You could add any functions you wish to this file to manipulate LAMMPS data however you wish.

The C++ driver does the same thing, except that it instantiates LAMMPS as an object first. Some of the functions in src/library.cpp can be invoked directly as methods within appropriate LAMMPS classes, which is what the driver does. Any public LAMMPS class method could be called from the driver this way. However the get/put functions are only implemented in src/library.cpp, so the C++ driver calls them as C-style functions.