# Running LAMMPS and Python in serial

To run a LAMMPS in serial, type these lines into Python interactively
from the `bench` directory:

``` python
from lammps import lammps
lmp = lammps()
lmp.file("in.lj")
```

Or put the same lines in the file `test.py` and run it as

``` bash
python3 test.py
```

Either way, you should see the results of running the `in.lj` benchmark
on a single processor appear on the screen, the same as if you had typed
something like:

``` bash
lmp_serial -in in.lj
```

# Running LAMMPS and Python in parallel with MPI

To run LAMMPS in parallel, assuming you have installed the
[mpi4py](https://mpi4py.readthedocs.io)\_ package as discussed
[python_install_mpi4py]{.title-ref}, create a `test.py` file containing
these lines:

``` python
from mpi4py import MPI
from lammps import lammps
lmp = lammps()
lmp.file("in.lj")
me = MPI.COMM_WORLD.Get_rank()
nprocs = MPI.COMM_WORLD.Get_size()
print("Proc %d out of %d procs has" % (me,nprocs),lmp)
MPI.Finalize()
```

You can run the script in parallel as:

``` bash
mpirun -np 4 python3 test.py
```

and you should see the same output as if you had typed

``` bash
mpirun -np 4 lmp_mpi -in in.lj
```

Note that without the mpi4py specific lines from `test.py`

``` python
from lammps import lammps
lmp = lammps()
lmp.file("in.lj")
```

running the script with `mpirun` on $P$ processors would lead to $P$
independent simulations to run parallel, each with a single processor.
Therefore, if you use the mpi4py lines and you see multiple LAMMPS
single processor outputs, mpi4py is not working correctly.

Also note that once you import the mpi4py module, mpi4py initializes MPI
for you, and you can use MPI calls directly in your Python script, as
described in the mpi4py documentation. The last line of your Python
script should be `MPI.finalize()`, to ensure MPI is shut down correctly.

# Running Python scripts

Note that any Python script (not just for LAMMPS) can be invoked in one
of several ways:

``` bash
python script.py
python -i script.py
./script.py
```

The last command requires that the first line of the script be something
like this:

``` bash
#!/usr/bin/python
```

or

``` bash
#!/usr/bin/env python
```

where the path in the first case needs to point to where you have Python
installed (the second option is workaround for when this may change),
and that you have made the script file executable:

``` bash
chmod +x script.py
```

Without the `-i` flag, Python will exit when the script finishes. With
the `-i` flag, you will be left in the Python interpreter when the
script finishes, so you can type subsequent commands. As mentioned
above, you can only run Python interactively when running Python on a
single processor, not in parallel.