lammps/couple/simple

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.