.. module:: ase.calculators.vasp

.. _vasp-calculator:

====
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: https://cms.mpi.univie.ac.at/vasp/
.. _Vasp 2.0: vasp2.html

.. note::
   A new VASP_ calculator is currently in BETA testing, see
   :mod:`~ase.calculators.vasp.vasp2`,
   which implements the calculator using the
   :class:`~ase.calculators.calculator.FileIOCalculator`.

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

.. _VASP vdW wiki: https://cms.mpi.univie.ac.at/vasp/vasp/vdW_DF_functional_Langreth_Lundqvist_et_al.html

The following environment variable can be used to automatically copy the
van der Waals kernel to the calculation directory. The kernel is needed for
vdW calculations, see `VASP vdW wiki`_, for more details. The kernel is looked
for, whenever ``luse_vdw=True``.

.. highlight:: bash

::

   $ export ASE_VASP_VDW=$HOME/<path-to-vdw_kernel.bindat-folder>

The environment variable :envvar:`ASE_VASP_VDW` should point to the folder where
the :file:`vdw_kernel.bindat` file is located.

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

The default setting used by the VASP interface is

.. autoclass:: Vasp

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. Defaults to
                                           None if ``gga`` set explicitly.
``setups``      ``str``    None            Additional setup option
``pp``          ``str``    Set by ``xc``   Pseudopotential (POTCAR) set
                           or ``gga``      used (LDA, PW91 or PBE).
``kpts``        various    `\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
``net_charge``  ``int``    None            Net charge per unit cell (as
                                           an alternative to specifying
                                           the total charge ``nelect``)
``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
``ldau_luj``    ``dict``                   LD(S)A+U parameters
==============  =========  ==============  ============================

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: https://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.

Exchange-correlation functionals
================================

The ``xc`` parameter is used to define a "recipe" of other parameters
including the pseudopotential set ``pp``.  It is possible to override
any parameters set with ``xc`` by setting them explicitly. For
example, the screening parameter of a HSE calculation might be
modified with

   >>> calc = ase.calculators.vasp.Vasp(xc='hse06', hfscreen=0.4)

The default pseudopotential set is potpaw_PBE unless ``xc`` or ``pp``
is set to ``pw91`` or ``lda``.

==========================  =====================================
``xc`` value                Parameters set
==========================  =====================================
lda, pbe, pw91              ``pp`` (``gga`` set implicity in POTCAR)
pbesol, revpbe, rpbe, am05  ``gga``
tpss, revtpss, m06l         ``metagga``
vdw-df, optpbe-vdw          ``gga``, ``luse_vdw``, ``aggac``
optb88-vdw, obptb86b-vdw    ``gga``, ``luse_vdw``, ``aggac``,
                            ``param1``, ``param2``
beef-vdw                    ``gga``, ``luse_vdw``, ``zab_vdw``
vdw-df2                     ``gga``, ``luse_vdw``, ``aggac``,
                            ``zab_vdw``
hf                          ``lhfcalc``, ``aexx``, ``aldac``,
                            ``aggac``
pbe0                        ``gga``, ``lhfcalc``
b3lyp                       ``gga``, ``lhfcalc``, ``aexx``, ``aggax``,
                            ``aggac``, ``aldac``
hse03, hse06, hsesol        ``gga``, ``lhfcalc``, ``hfscreen``
==========================  =====================================

It is possible for the user to temporarily add their own ``xc``
recipes without modifying ASE, by updating a dictionary. For example,
to implement a hybrid PW91 calculation:

.. code-block:: python

   from ase.calculators.vasp import Vasp
   Vasp.xc_defaults['pw91_0'] = {'gga': '91', 'lhfcalc': True}

   calc = Vasp(xc='PW91_0')

Note that the dictionary keys must be *lower case*, while the ``xc``
parameter is case-insensitive when used.


Setups
======

For many elements, VASP is distributed with a choice of
pseudopotential setups. These may be hard/soft variants of the
pseudopotential or include additional valence electrons.
Three base setups are provided:

    minimal (default):
        If a PAW folder exists with the same name as the element,
        this will be used. For the other elements, the PAW setup
        with the least electrons has been chosen.
    recommended:
        corresponds to the `table of recommended PAW setups <https://cms.mpi.univie.ac.at/vasp/vasp/Recommended_PAW_potentials_DFT_calculations_using_vasp_5_2.html>`_ supplied by the VASP developers.
    materialsproject:
        corresponds to the `Materials Project recommended PAW setups <https://wiki.materialsproject.org/Pseudopotentials_Choice>`_.
    gw:
        corresponds to the `table of recommended setups for GW <https://cms.mpi.univie.ac.at/vasp/vasp/Recommended_GW_PAW_potentials_vasp_5_2.html>`_ supplied by the VASP developers.

Where elements are missing from the default sets, the Vasp Calculator
will attempt to use a setup folder with the same name as the element.
A default setup may be selected with the ``setups`` keyword:

.. code-block:: python

    from ase.calculators.vasp import Vasp
    calc = Vasp(setups='recommended')

To use an alternative setup for all instances of an element, use the
dictionary form of ``setups`` to provide the characters which need
to be added to the element name, e.g.

.. code-block:: python

   calc = Vasp(xc='PBE', setups={'Li': '_sv'})

will use the ``Li_sv`` all-electron pseudopotential for all Li atoms.

To apply special setups to individual atoms, identify them by their
zero-indexed number in the atom list and use the full setup name. For
example,

.. code-block:: python

   calc = Vasp(xc='PBE', setups={3: 'Ga_d'})

will treat the Ga atom in position 3 (i.e. the fourth atom) of the
atoms object as special, with an additional 10 d-block valence
electrons, while other Ga atoms use the default 3-electron setup and
other elements use their own default setups. The positional index may
be quoted as a string (e.g. ``{'3': 'Ga_d'}``).

These approaches may be combined by using the 'base' key to access a
default set, e.g.

.. code-block:: python

   calc = Vasp(xc='PBE', setups={'base': 'recommended', 'Li': '', 4: 'H.5'})

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.

Brillouin-zone sampling
=======================

Brillouin-zone sampling is controlled by the parameters ``kpts``,
``gamma`` and ``reciprocal``, and may also be set with the VASP
parameters ``kspacing`` and ``kgamma``.

Single-parameter schemes
------------------------
A **k**-point mesh may be set using a single value in one of two ways:

Scalar ``kpts``
  If ``kpts`` is declared as a scalar (i.e. a float or an int), an
  appropriate KPOINTS file will be written. The value of ``kpts`` will
  be used to set a length cutoff for the Gamma-centered “Automatic”
  scheme provided by VASP. (See `first example
  <https://cms.mpi.univie.ac.at/vasp/vasp/Automatic_k_mesh_generation.html>`_
  in VASP manual.)

KSPACING and KGAMMA
  Alternatively, the **k**-point density can be set in the INCAR file with
  these flags as `described in the VASP manual
  <https://cms.mpi.univie.ac.at/vasp/vasp/KSPACING_tag_KGAMMA_tag.html>`_. If
  ``kspacing`` is set, the ASE calculator will not write out a KPOINTS
  file.

Three-parameter scheme
----------------------

Brillouin-zone sampling can also be specified by defining a number of
subdivisions for each reciprocal lattice vector.

This is the `second “Automatic” scheme <https://cms.mpi.univie.ac.at/vasp/vasp/Automatic_k_mesh_generation.html>`_ described in the VASP manual.
In the ASE calculator, it is used by setting ``kpts`` to a sequence of three ``int`` values, e.g. ``[2, 2, 3]``.
If ``gamma` is set to ``True``, the mesh will be centred at the `\Gamma`-point;
otherwise, a regular Monkhorst-Pack grid is used, which may or may not include the `\Gamma`-point.

In VASP it is possible to define an automatic grid and shift the origin point.
This function is not currently included in the ASE calculator. The same result can be achieved by using :func:`ase.dft.kpoints.monkhorst_pack` to generate an explicit list of **k**-points (see below) and simply adding a constant vector to the matrix.
For example,

.. code-block:: python

    import ase.dft.kpoints
    kpts = ase.dft.kpoints.monkhorst_pack([2, 2, 1]) + [0.25, 0.25, 0.5]

creates an acceptable ``kpts`` array with the values

.. code-block:: python

  array([[ 0. ,  0. ,  0.5],
         [ 0. ,  0.5,  0.5],
         [ 0.5,  0. ,  0.5],
         [ 0.5,  0.5,  0.5]])

However, this method will prevent VASP from using symmetry to reduce the number of calculated points.

Explicitly listing the **k**-points
-----------------------------------
If an *n*-by-3 or *n*-by-4 array is used for ``kpts``,
this is interpreted as a list of *n* explicit **k**-points and an appropriate KPOINTS file is generated.
The fourth column, if provided, sets the sample weighting of each point.
Otherwise, all points are weighted equally.

Usually in these cases it is desirable to set the ``reciprocal`` parameter to ``True``,
so that the **k**-point vectors are given relative to the reciprocal lattice.
Otherwise, they are taken as being in Cartesian space.

Band structure paths
--------------------
VASP provides a “line-mode” for the generation of band-structure paths.
While this is not directly supported by ASE, relevant functionality exists in the :mod:`ase.dft.kpoints` module.
For example:

.. code-block:: python

    import ase.build
    from ase.dft.kpoints import bandpath

    si = ase.build.bulk('Si')
    kpts, x_coords, x_special_points = bandpath('GXL', si.cell, npoints=20)

returns an acceptable ``kpts`` array (for use with ``reciprocal=True``) as well as plotting information.

LD(S)A+U
========
The VASP +U corrections can be turned on using the default VASP parameters explicitly, by manually setting
the ``ldaul``, ``ldauu`` and ``ldauj`` parameters, as well as enabling ``ldau``.

However, ASE offers a convenient ASE specific keyword to enable these, by using a dictionary construction, through the
``ldau_luj`` keyword. If the user does not explicitly set ``ldau=False``, then ``ldau=True`` will automatically
be set if ``ldau_luj`` is set.
For example:

.. code-block:: python

    calc = Vasp(ldau_luj={'Si': {'L': 1, 'U': 3, 'J': 0}})

will set ``U=3`` on the Si p-orbitals, and will automatically set ``ldau=True`` as well.


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

New Calculator
==============

A new VASP_ calculator is currently in BETA testing, see
:mod:`~ase.calculators.vasp.vasp2`, which implements the calculator using the
:class:`~ase.calculators.calculator.FileIOCalculator`.

.. toctree::

   vasp2
