.. module:: vasp

====
VASP
====

Introduction
============

VASP_ is a density-functional theory code using pseudopotentials or 
the projector-augmented wave method and a plane wave basis set. This 
interface makes it possible to use VASP_ as a calculator in ASE, and 
also to use ASE as a post-processor for an already performed VASP_
calculation.


.. _VASP: http://cms.mpi.univie.ac.at/vasp/



Environment variables
=====================

You need to write a script called :file:`run_vasp.py` containing
something like this::

  import os
  exitcode = os.system('vasp')

The environment variable :envvar:`VASP_SCRIPT` must point to that file.

A directory containing the pseudopotential directories :file:`potpaw` 
(LDA XC) :file:`potpaw_GGA` (PW91 XC) and :file:`potpaw_PBE` (PBE XC)
is also needed, and it is to be put in the environment variable
:envvar:`VASP_PP_PATH`.

Set both environment variables in your shell configuration file:

.. highlight:: bash
 
::

  $ export VASP_SCRIPT=$HOME/vasp/run_vasp.py
  $ export VASP_PP_PATH=$HOME/vasp/mypps

.. highlight:: python



VASP Calculator
=============== 

The default setting used by the VASP interface is

.. class:: Vasp(restart=None, xc='PW91', setups=None, kpts=(1,1,1), gamma=None)

Below follows a list with a selection of parameters

==============  =========  ==============  ============================
keyword         type       default value   description
==============  =========  ==============  ============================
``restart``     ``bool``   None            Restart old calculation or
                                           use ASE for post-processing
``xc``          ``str``    ``'PW91'``      XC-functional
``setups``      ``str``    None            Additional setup option
``kpts``        *seq*      `\Gamma`-point  **k**-point sampling
``gamma``       ``bool``   None            `\Gamma`-point centered 
                                           **k**-point sampling
``reciprocal``  ``bool``   None		   Use reciprocal units if
                                           **k**-points are specified 
                                           explicitly
``prec``        ``str``                    Accuracy of calculation
``encut``       ``float``                  Kinetic energy cutoff
``ediff``       ``float``                  Convergence break condition
                                           for SC-loop.
``nbands``      ``int``                    Number of bands
``algo``        ``str``                    Electronic minimization 
                                           algorithm
``ismear``      ``int``                    Type of smearing
``sigma``       ``float``                  Width of smearing
``nelm``        ``int``                    Maximum number of
                                           SC-iterations
==============  =========  ==============  ============================

*seq*: A sequence of three ``int``'s.

For parameters in the list without default value given, VASP will set 
the default value. Most of the parameters used in the VASP :file:`INCAR` file 
are allowed keywords. See the official `VASP manual`_ for more details.

.. _VASP manual: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html


.. note:: 
   
   Parameters can be changed after the calculator has been constructed
   by using the :meth:`~ase.calculators.vasp.Vasp.set` method:

   >>> calc.set(prec='Accurate', ediff=1E-5)

   This would set the precision to Accurate and the break condition for 
   the electronic SC-loop to ``1E-5`` eV.


Spin-polarized calculation
==========================

If the atoms object has non-zero magnetic moments, a spin-polarized calculation
will be performed by default.

Here follows an example how to calculate the total magnetic moment of a sodium
chloride molecule.
  
.. literalinclude:: NaCl.py

In this example the initial magnetic moments are assigned to the atoms
when defining the Atoms object. The calculator will detect that at least
one of the atoms has a non-zero magnetic moment and a spin-polarized
calculation will automatically be performed. The ASE generated :file:`INCAR`
file will look like:

.. literalinclude:: INCAR_NaCl


.. note:: 
   
   It is also possible to manually tell the calculator to perform a 
   spin-polarized calculation:

   >>> calc.set(ispin=2)

   This can be useful for continuation jobs, where the initial magnetic 
   moment is read from the WAVECAR file.


Restart old calculation
=======================

To continue an old calculation which has been performed without the interface
use the ``restart`` parameter when constructing the calculator

>>> calc = Vasp(restart=True)

Then the calculator will read atomic positions from the :file:`CONTCAR` file,
physical quantities from the :file:`OUTCAR` file, **k**-points from the
:file:`KPOINTS` file and parameters from the :file:`INCAR` file. 

.. note::

   Only Monkhorst-Pack and \Gamma-centered **k**-point sampling are supported 
   for restart at the moment. Some :file:`INCAR` parameters may not be
   implemented for restart yet. Please report any problems to the ASE mailing
   list.

The ``restart`` parameter can be used , as the name suggest to continue a job from where a
previous calculation finished. Furthermore, it can be used to extract data from
an already performed calculation. For example, to get the total potential energy
of the sodium chloride molecule in the previous section, without performing any additional
calculations, in the directory of the previous calculation do:

>>> calc = Vasp(restart=True)
>>> atoms = calc.get_atoms()
>>> atoms.get_potential_energy()
-4.7386889999999999
