.. _mpi4py_url: https://mpi4py.readthedocs.io/

.. _python_create_lammps:

Creating or deleting a LAMMPS object
====================================

With the Python interface the creation of a :cpp:class:`LAMMPS
<LAMMPS_NS::LAMMPS>` instance is included in the constructors for the
:py:class:`lammps <lammps.lammps>`, :py:class:`PyLammps <lammps.PyLammps>`,
and :py:class:`IPyLammps <lammps.IPyLammps>` classes.
Internally it will call either :cpp:func:`lammps_open` or :cpp:func:`lammps_open_no_mpi` from the C
library API to create the class instance.

All arguments are optional.  The *name* argument allows loading a
LAMMPS shared library that is named ``liblammps_machine.so`` instead of
the default name of ``liblammps.so``.  In most cases the latter will be
installed or used.  The *ptr* argument is for use of the
:py:mod:`lammps` module from inside a LAMMPS instance, e.g. with the
:doc:`python <python>` command, where a pointer to the already existing
:cpp:class:`LAMMPS <LAMMPS_NS::LAMMPS>` class instance can be passed
to the Python class and used instead of creating a new instance.  The
*comm* argument may be used in combination with the `mpi4py <mpi4py_url_>`_
module to pass an MPI communicator to LAMMPS and thus it is possible
to run the Python module like the library interface on a subset of the
MPI ranks after splitting the communicator.


Here are simple examples using all three Python interfaces:

.. tabs::

   .. tab:: lammps API

      .. code-block:: python

         from lammps import lammps

         # NOTE: argv[0] is set by the lammps class constructor
         args = ["-log", "none"]

         # create LAMMPS instance
         lmp = lammps(cmdargs=args)

         # get and print numerical version code
         print("LAMMPS Version: ", lmp.version())

         # explicitly close and delete LAMMPS instance (optional)
         lmp.close()

   .. tab:: PyLammps API

      The :py:class:`PyLammps <lammps.PyLammps>` class is a wrapper around the
      :py:class:`lammps <lammps.lammps>` class and all of its lower level functions.
      By default, it will create a new instance of :py:class:`lammps <lammps.lammps>` passing
      along all arguments to the constructor of :py:class:`lammps <lammps.lammps>`.

      .. code-block:: python

         from lammps import PyLammps

         # NOTE: argv[0] is set by the lammps class constructor
         args = ["-log", "none"]

         # create LAMMPS instance
         L = PyLammps(cmdargs=args)

         # get and print numerical version code
         print("LAMMPS Version: ", L.version())

         # explicitly close and delete LAMMPS instance (optional)
         L.close()

      :py:class:`PyLammps <lammps.PyLammps>` objects can also be created on top of an existing
      :py:class:`lammps <lammps.lammps>` object:

      .. code-block:: Python

         from lammps import lammps, PyLammps
         ...
         # create LAMMPS instance
         lmp = lammps(cmdargs=args)

         # create PyLammps instance using previously created LAMMPS instance
         L = PyLammps(ptr=lmp)

      This is useful if you have to create the :py:class:`lammps <lammps.lammps>`
      instance is a specific way, but want to take advantage of the
      :py:class:`PyLammps <lammps.PyLammps>` interface.

   .. tab:: IPyLammps API

      The :py:class:`IPyLammps <lammps.IPyLammps>` class is an extension of the
      :py:class:`PyLammps <lammps.PyLammps>` class. It has the same construction behavior. By
      default, it will create a new instance of :py:class:`lammps` passing
      along all arguments to the constructor of :py:class:`lammps`.

      .. code-block:: python

         from lammps import IPyLammps

         # NOTE: argv[0] is set by the lammps class constructor
         args = ["-log", "none"]

         # create LAMMPS instance
         L = IPyLammps(cmdargs=args)

         # get and print numerical version code
         print("LAMMPS Version: ", L.version())

         # explicitly close and delete LAMMPS instance (optional)
         L.close()

      You can also initialize IPyLammps on top of an existing :py:class:`lammps` or :py:class:`PyLammps` object:

      .. code-block:: Python

         from lammps import lammps, IPyLammps
         ...
         # create LAMMPS instance
         lmp = lammps(cmdargs=args)

         # create PyLammps instance using previously created LAMMPS instance
         L = PyLammps(ptr=lmp)

      This is useful if you have to create the :py:class:`lammps <lammps.lammps>`
      instance is a specific way, but want to take advantage of the
      :py:class:`IPyLammps <lammps.IPyLammps>` interface.

In all of the above cases, same as with the :ref:`C library API <lammps_c_api>`, this will use the
``MPI_COMM_WORLD`` communicator for the MPI library that LAMMPS was
compiled with.

The :py:func:`lmp.close() <lammps.lammps.close()>` call is
optional since the LAMMPS class instance will also be deleted
automatically during the :py:class:`lammps <lammps.lammps>` class
destructor.

Note that you can create multiple LAMMPS objects in your Python
script, and coordinate and run multiple simulations, e.g.

.. code-block:: Python

   from lammps import lammps
   lmp1 = lammps()
   lmp2 = lammps()
   lmp1.file("in.file1")
   lmp2.file("in.file2")