lammps/examples/COUPLE/multiple

This directory has a C++ code multiple.cpp which illustrates how a
driver code can create multiple instantiations of LAMMPS to run
independent simulations simultaneously, on a set of processors
allocated by MPI.

Once you have built LAMMPS as a library (see examples/COUPLE/README),
you can then build the driver codes with compile lines like these,
which include paths to the LAMMPS library interface, MPI (an installed
MPICH in this case), and FFTW (assuming you built LAMMPS as a library
with its PPPM solver).

g++ -I/home/sjplimp/lammps/src -c multiple.cpp
g++ -L/home/sjplimp/lammps/src multiple.o \
    -llammps -lfftw -lmpich -lmpl -lpthread -o multiple

If you then run this command:

% mpirun -np 8 multiple 4 in.chain 1.0 0.5

you will create 4 instances of LAMMPS, each running on 2 of the
allocated 8 processors.  The final two arguments are the base
temperature and a delta temperature added by each instance.  The
provided input script in.chain uses those temperatures, set as a
variable "t" when each instance of LAMMPS is created.

After a few seconds you should get this output to the screen:

Instance 1, final temp = 1.00469
Instance 2, final temp = 1.50395
Instance 3, final temp = 1.99873
Instance 4, final temp = 2.50862

indicating the runs equilibrated to 4 temperatures separated by 0.5
(in LJ units).  There should also be 4 screen files (screen.N) and 4
log files (log.lammps.N) from the individual LAMMPS runs.