lammps/examples/couple

This directory has a 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.

Umbrella.cpp is the simple driving code that will call LAMMPS.

LAMMPS must first be built as a library.  See the "Making LAMMPS"
section of Section_start.html in the documentation for info on how to
do this.  Basically, you type something like

make makelib
make -f Makefile.lib g++

in the LAMMPS src directory to create liblmp_g++.a

You can then build the umbrella code with a compile line something
like this, which includes paths to the LAMMPS library interface, MPI,
and FFTW.

g++ -I/home/sjplimp/tools/mpich/include -I/home/sjplimp/lammps/src
    -L/home/sjplimp/lammps/src -L/home/sjplimp/tools/mpich/lib
    -L/home/sjplimp/tools/fftw/lib umbrella.cpp
    -llmp_g++ -lfftw -lmpich -o umbrella

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

mpirun -np Q umbrella P in.lj

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

Umbrella 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.

Umbrella 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.