Update python docs
This commit is contained in:
Richard Berger
@ -73,12 +73,13 @@ Creating or deleting a LAMMPS object
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************************************
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With the Python interface the creation of a :cpp:class:`LAMMPS
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<LAMMPS_NS::LAMMPS>` instance is included in the constructor for the
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:py:func:`lammps <lammps.lammps>` class. Internally it will call either
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:cpp:func:`lammps_open` or :cpp:func:`lammps_open_no_mpi` from the C
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<LAMMPS_NS::LAMMPS>` instance is included in the constructors for the
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:py:meth:`lammps <lammps.lammps.__init__()>`, :py:meth:`PyLammps <lammps.PyLammps.__init__()>`,
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and :py:meth:`PyLammps <lammps.IPyLammps.__init__()>` classes.
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Internally it will call either :cpp:func:`lammps_open` or :cpp:func:`lammps_open_no_mpi` from the C
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library API to create the class instance.
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All arguments are optional. The *name* argument is to allow loading a
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All arguments are optional. The *name* argument allows loading a
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LAMMPS shared library that is named ``liblammps_machine.so`` instead of
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the default name of ``liblammps.so``. In most cases the latter will be
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installed or used. The *ptr* argument is for use of the
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@ -89,22 +90,111 @@ to the Python class and used instead of creating a new instance. The
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*comm* argument may be used in combination with the `mpi4py <mpi4py_url_>`_
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module to pass an MPI communicator to LAMMPS and thus it is possible
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to run the Python module like the library interface on a subset of the
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MPI ranks after splitting the communicator. Here is a simple example:
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MPI ranks after splitting the communicator.
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.. code-block:: python
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from lammps import lammps
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Here are simple examples using all three Python interfaces:
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# NOTE: argv[0] is set by the Python module
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args = ["-log", "none"]
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# create LAMMPS instance
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lmp = lammps(cmdargs=args)
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# get and print numerical version code
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print("LAMMPS Version: ", lmp.version())
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# explicitly close and delete LAMMPS instance (optional)
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lmp.close()
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.. tabs::
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Same as with the :ref:`C library API <lammps_c_api>` this will use the
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.. tab:: lammps API
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.. code-block:: python
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from lammps import lammps
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# NOTE: argv[0] is set by the lammps class constructor
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args = ["-log", "none"]
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# create LAMMPS instance
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lmp = lammps(cmdargs=args)
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# get and print numerical version code
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print("LAMMPS Version: ", lmp.version())
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# explicitly close and delete LAMMPS instance (optional)
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lmp.close()
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.. tab:: PyLammps API
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The :py:class:`PyLammps` class is a wrapper around the
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:py:class:`lammps` class and all of its lower level functions.
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By default, it will create a new instance of :py:class:`lammps` passing
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along all arguments to the constructor of :py:class:`lammps`.
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.. code-block:: python
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from lammps import PyLammps
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# NOTE: argv[0] is set by the lammps class constructor
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args = ["-log", "none"]
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# create LAMMPS instance
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L = PyLammps(cmdargs=args)
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# get and print numerical version code
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print("LAMMPS Version: ", L.version())
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# explicitly close and delete LAMMPS instance (optional)
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L.close()
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:py:class:`PyLammps` objects can also be created on top of an existing :py:class:`lammps` object:
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.. code-block:: Python
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from lammps import lammps, PyLammps
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# NOTE: argv[0] is set by the lammps class constructor
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args = ["-log", "none"]
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# create LAMMPS instance
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lmp = lammps(cmdargs=args)
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# create PyLammps instance using previously created LAMMPS instance
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L = PyLammps(ptr=lmp)
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# get and print numerical version code
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print("LAMMPS Version: ", L.version())
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# explicitly close and delete LAMMPS instance (optional)
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L.close()
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This is useful if you have to create the :py:class:`lammps <lammps.lammps>`
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instance is a specific way, but want to take advantage of the
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:py:class:`PyLammps <lammps.PyLammps>` interface.
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.. tab:: IPyLammps API
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The :py:class:`IPyLammps` class is an extension of the
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:py:class:`PyLammps` class. It has the same construction behavior. By
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default, it will create a new instance of :py:class:`lammps` passing
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along all arguments to the constructor of :py:class:`lammps`.
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.. code-block:: python
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from lammps import IPyLammps
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# NOTE: argv[0] is set by the lammps class constructor
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args = ["-log", "none"]
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# create LAMMPS instance
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L = IPyLammps(cmdargs=args)
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# get and print numerical version code
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print("LAMMPS Version: ", L.version())
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# explicitly close and delete LAMMPS instance (optional)
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L.close()
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You can also initialize IPyLammps on top of an existing :py:class:`lammps` or :py:class:`PyLammps` object:
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.. code-block:: Python
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from lammps import lammps, IPyLammps
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# NOTE: argv[0] is set by the lammps class constructor
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args = ["-log", "none"]
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# create LAMMPS instance
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lmp = lammps(cmdargs=args)
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# create IPyLammps instance using previously created LAMMPS instance
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L = IPyLammps(ptr=lmp)
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# get and print numerical version code
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print("LAMMPS Version: ", L.version())
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# explicitly close and delete LAMMPS instance (optional)
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L.close()
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This is useful if you have to create the :py:class:`lammps <lammps.lammps>`
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instance is a specific way, but want to take advantage of the
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:py:class:`IPyLammps <lammps.IPyLammps>` interface.
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In all of the above cases, same as with the :ref:`C library API <lammps_c_api>`, this will use the
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``MPI_COMM_WORLD`` communicator for the MPI library that LAMMPS was
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compiled with. The :py:func:`lmp.close() <lammps.lammps.close>` call is
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optional since the LAMMPS class instance will also be deleted
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@ -114,39 +204,107 @@ destructor.
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Executing LAMMPS commands
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*************************
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Once an instance of the :py:class:`lammps <lammps.lammps>` class is
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created, there are multiple ways to "feed" it commands. In a way that is
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not very different from running a LAMMPS input script, except that
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Python has many more facilities for structured programming than the
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LAMMPS input script syntax. Furthermore it is possible to "compute"
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what the next LAMMPS command should be. Same as in the equivalent `C
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library functions <pg_lib_execute>`, commands can be read from a file, a
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single string, a list of strings and a block of commands in a single
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multi-line string. They are processed under the same boundary conditions
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as the C library counterparts. The example below demonstrates the use
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of :py:func:`lammps.file`, :py:func:`lammps.command`,
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:py:func:`lammps.commands_list`, and :py:func:`lammps.commands_string`:
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Once an instance of the :py:class:`lammps`, :py:class:`PyLammps`, or
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:py:class:`IPyLammps` class is created, there are multiple ways to "feed" it
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commands. In a way that is not very different from running a LAMMPS input
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script, except that Python has many more facilities for structured
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programming than the LAMMPS input script syntax. Furthermore it is possible
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to "compute" what the next LAMMPS command should be.
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.. code-block:: python
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.. tabs::
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from lammps import lammps
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.. tab:: lammps API
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lmp = lammps()
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# read commands from file 'in.melt'
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lmp.file('in.melt')
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# issue a single command
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lmp.command('variable zpos index 1.0')
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# create 10 groups with 10 atoms each
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cmds = ["group g{} id {}:{}".format(i,10*i+1,10*(i+1)) for i in range(10)]
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lmp.commands_list(cmds)
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# run commands from a multi-line string
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block = """
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clear
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region box block 0 2 0 2 0 2
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create_box 1 box
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create_atoms 1 single 1.0 1.0 ${zpos}
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"""
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lmp.commands_string(block)
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Same as in the equivalent
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`C library functions <pg_lib_execute>`, commands can be read from a file, a
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single string, a list of strings and a block of commands in a single
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multi-line string. They are processed under the same boundary conditions
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as the C library counterparts. The example below demonstrates the use
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of :py:func:`lammps.file`, :py:func:`lammps.command`,
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:py:func:`lammps.commands_list`, and :py:func:`lammps.commands_string`:
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.. code-block:: python
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from lammps import lammps
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lmp = lammps()
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# read commands from file 'in.melt'
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lmp.file('in.melt')
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# issue a single command
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lmp.command('variable zpos index 1.0')
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# create 10 groups with 10 atoms each
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cmds = ["group g{} id {}:{}".format(i,10*i+1,10*(i+1)) for i in range(10)]
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lmp.commands_list(cmds)
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# run commands from a multi-line string
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block = """
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clear
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region box block 0 2 0 2 0 2
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create_box 1 box
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create_atoms 1 single 1.0 1.0 ${zpos}
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"""
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lmp.commands_string(block)
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.. tab:: PyLammps/IPyLammps API
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Unlike the lammps API, the PyLammps/IPyLammps APIs allow running LAMMPS
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commands by calling equivalent member functions.
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For instance, the following LAMMPS command:
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.. code-block:: LAMMPS
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region box block 0 10 0 5 -0.5 0.5
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In the original interface this command can be executed with the following
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Python code if *L* was a lammps instance:
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.. code-block:: Python
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L.command("region box block 0 10 0 5 -0.5 0.5")
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With the PyLammps interface, any command can be split up into arbitrary parts
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separated by white-space, passed as individual arguments to a :code:`region` method.
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.. code-block:: Python
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L.region("box block", 0, 10, 0, 5, -0.5, 0.5)
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Note that each parameter is set as Python literal floating-point number. In the
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PyLammps interface, each command takes an arbitrary parameter list and transparently
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merges it to a single command string, separating individual parameters by white-space.
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The benefit of this approach is avoiding redundant command calls and easier
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parameterization. In the original interface parameterization needed to be done
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manually by creating formatted strings.
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.. code-block:: Python
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L.command("region box block %f %f %f %f %f %f" % (xlo, xhi, ylo, yhi, zlo, zhi))
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In contrast, methods of PyLammps accept parameters directly and will convert
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them automatically to a final command string.
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.. code-block:: Python
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L.region("box block", xlo, xhi, ylo, yhi, zlo, zhi)
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Using these facilities, the example shown for the lammps API can be rewritten as follows:
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.. code-block:: python
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from lammps import PyLammps
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L = PyLammps()
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# read commands from file 'in.melt'
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L.file('in.melt')
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# issue a single command
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L.variable('zpos', 'index', 1.0)
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# create 10 groups with 10 atoms each
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for i in range(10):
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L.group(f"g{i}", "id", f"{10*i+1}:{10*(i+1)}")
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L.clear()
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L.region("box block", 0, 2, 0, 2, 0, 2)
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L.create_box(1, "box")
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L.create_atoms(1, "single", 1.0, 1.0, "${zpos}")
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----------
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@ -176,239 +334,17 @@ simpler, more "Pythonic" interface to common LAMMPS functionality. LAMMPS data
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structures are exposed through objects and properties. This makes Python scripts
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shorter and more concise.
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Creating a new instance of PyLammps
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-----------------------------------
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To create a PyLammps object you need to first import the class from the lammps
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module. By using the default constructor, a new :py:class:`lammps <lammps.lammps>` instance is created.
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.. code-block:: Python
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from lammps import PyLammps
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L = PyLammps()
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You can also initialize PyLammps on top of this existing :py:class:`lammps <lammps.lammps>` object:
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.. code-block:: Python
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from lammps import lammps, PyLammps
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lmp = lammps()
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L = PyLammps(ptr=lmp)
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This is useful if you have create the :py:class:`lammps <lammps.lammps>`
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instance is a specific way, but want to take advantage of the
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:py:class:`PyLammps <lammps.PyLammps>` interface.
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Commands
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--------
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Sending a LAMMPS command with the existing library interfaces is done using
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the command method of the lammps object instance.
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For instance, let's take the following LAMMPS command:
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.. code-block:: LAMMPS
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region box block 0 10 0 5 -0.5 0.5
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In the original interface this command can be executed with the following
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Python code if *L* was a lammps instance:
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.. code-block:: Python
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L.command("region box block 0 10 0 5 -0.5 0.5")
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With the PyLammps interface, any command can be split up into arbitrary parts
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separated by white-space, passed as individual arguments to a region method.
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.. code-block:: Python
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L.region("box block", 0, 10, 0, 5, -0.5, 0.5)
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Note that each parameter is set as Python literal floating-point number. In the
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PyLammps interface, each command takes an arbitrary parameter list and transparently
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merges it to a single command string, separating individual parameters by white-space.
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The benefit of this approach is avoiding redundant command calls and easier
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parameterization. In the original interface parameterization needed to be done
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manually by creating formatted strings.
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.. code-block:: Python
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L.command("region box block %f %f %f %f %f %f" % (xlo, xhi, ylo, yhi, zlo, zhi))
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In contrast, methods of PyLammps accept parameters directly and will convert
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them automatically to a final command string.
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.. code-block:: Python
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L.region("box block", xlo, xhi, ylo, yhi, zlo, zhi)
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System state
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------------
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In addition to dispatching commands directly through the PyLammps object, it
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also provides several properties which allow you to query the system state.
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:py:attr:`lammps.PyLammps.system`
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Is a dictionary describing the system such as the bounding box or number of atoms
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L.system.xlo, L.system.xhi
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bounding box limits along x-axis
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L.system.ylo, L.system.yhi
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bounding box limits along y-axis
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L.system.zlo, L.system.zhi
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bounding box limits along z-axis
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L.communication
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configuration of communication subsystem, such as the number of threads or processors
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L.communication.nthreads
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number of threads used by each LAMMPS process
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L.communication.nprocs
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number of MPI processes used by LAMMPS
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L.fixes
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List of fixes in the current system
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L.computes
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List of active computes in the current system
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L.dump
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List of active dumps in the current system
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L.groups
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List of groups present in the current system
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Working with LAMMPS variables
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-----------------------------
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LAMMPS variables can be both defined and accessed via the PyLammps interface.
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To define a variable you can use the :doc:`variable <variable>` command:
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.. code-block:: Python
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L.variable("a index 2")
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A dictionary of all variables is returned by L.variables
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you can access an individual variable by retrieving a variable object from the
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L.variables dictionary by name
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.. code-block:: Python
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a = L.variables['a']
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The variable value can then be easily read and written by accessing the value
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property of this object.
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.. code-block:: Python
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print(a.value)
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a.value = 4
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Retrieving the value of an arbitrary LAMMPS expressions
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-------------------------------------------------------
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LAMMPS expressions can be immediately evaluated by using the eval method. The
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passed string parameter can be any expression containing global thermo values,
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variables, compute or fix data.
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.. code-block:: Python
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result = L.eval("ke") # kinetic energy
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result = L.eval("pe") # potential energy
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result = L.eval("v_t/2.0")
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Accessing atom data
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-------------------
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All atoms in the current simulation can be accessed by using the L.atoms list.
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Each element of this list is an object which exposes its properties (id, type,
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position, velocity, force, etc.).
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.. code-block:: Python
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# access first atom
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L.atoms[0].id
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L.atoms[0].type
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# access second atom
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L.atoms[1].position
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L.atoms[1].velocity
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L.atoms[1].force
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Some properties can also be used to set:
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.. code-block:: Python
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# set position in 2D simulation
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L.atoms[0].position = (1.0, 0.0)
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# set position in 3D simulation
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L.atoms[0].position = (1.0, 0.0, 1.)
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Evaluating thermo data
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||||
----------------------
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Each simulation run usually produces thermo output based on system state,
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computes, fixes or variables. The trajectories of these values can be queried
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after a run via the L.runs list. This list contains a growing list of run data.
|
||||
The first element is the output of the first run, the second element that of
|
||||
the second run.
|
||||
|
||||
.. code-block:: Python
|
||||
|
||||
L.run(1000)
|
||||
L.runs[0] # data of first 1000 time steps
|
||||
|
||||
L.run(1000)
|
||||
L.runs[1] # data of second 1000 time steps
|
||||
|
||||
Each run contains a dictionary of all trajectories. Each trajectory is
|
||||
accessible through its thermo name:
|
||||
|
||||
.. code-block:: Python
|
||||
|
||||
L.runs[0].thermo.Step # list of time steps in first run
|
||||
L.runs[0].thermo.Ke # list of kinetic energy values in first run
|
||||
|
||||
Together with matplotlib plotting data out of LAMMPS becomes simple:
|
||||
|
||||
.. code-block:: Python
|
||||
|
||||
import matplotlib.plot as plt
|
||||
steps = L.runs[0].thermo.Step
|
||||
ke = L.runs[0].thermo.Ke
|
||||
plt.plot(steps, ke)
|
||||
|
||||
Error handling with PyLammps
|
||||
----------------------------
|
||||
|
||||
Compiling the shared library with C++ exception support provides a better error
|
||||
handling experience. Without exceptions the LAMMPS code will terminate the
|
||||
current Python process with an error message. C++ exceptions allow capturing
|
||||
them on the C++ side and rethrowing them on the Python side. This way you
|
||||
can handle LAMMPS errors through the Python exception handling mechanism.
|
||||
|
||||
.. warning::
|
||||
|
||||
Capturing a LAMMPS exception in Python can still mean that the
|
||||
current LAMMPS process is in an illegal state and must be terminated. It is
|
||||
advised to save your data and terminate the Python instance as quickly as
|
||||
possible.
|
||||
|
||||
.. autoclass:: lammps.PyLammps
|
||||
:members:
|
||||
|
||||
.. autoclass:: lammps.AtomList
|
||||
:members:
|
||||
|
||||
.. autoclass:: lammps.Atom
|
||||
:members:
|
||||
|
||||
.. autoclass:: lammps.Atom2D
|
||||
:members:
|
||||
|
||||
----------
|
||||
|
||||
@ -463,3 +399,19 @@ and the :py:class:`NeighList <lammps.NeighList>` class:
|
||||
:members:
|
||||
:no-undoc-members:
|
||||
|
||||
|
||||
LAMMPS error handling in Python
|
||||
*******************************
|
||||
|
||||
Compiling the shared library with C++ exception support provides a better error
|
||||
handling experience. Without exceptions the LAMMPS code will terminate the
|
||||
current Python process with an error message. C++ exceptions allow capturing
|
||||
them on the C++ side and rethrowing them on the Python side. This way
|
||||
LAMMPS errors can be handled through the Python exception handling mechanism.
|
||||
|
||||
.. warning::
|
||||
|
||||
Capturing a LAMMPS exception in Python can still mean that the
|
||||
current LAMMPS process is in an illegal state and must be terminated. It is
|
||||
advised to save your data and terminate the Python instance as quickly as
|
||||
possible.
|
||||
|
||||
Reference in New Issue
Block a user