.. _vasp_interface:

VASP & phonopy calculation
==================================

Pre-process
~~~~~~~~~~~~

The input stureture of ``POSCAR`` (:ref:`this <example_POSCAR1>`) is
used as an example here. Most files are found at `SiO2-HP example
<https://github.com/phonopy/phonopy/tree/master/example/SiO2-HP/>`_.

In the pre-process, supercell structures with (or without)
displacements are created from a unit cell fully consiering crystal
symmetry.

To obtain supercells (:math:`2\times 2\times 3`) with displacements,
run phonopy::

   phonopy -d --dim="2 2 3"

You should find the files, ``SPOSCAR``, ``phonopy_disp.yaml``, and
``POSCAR-{number}`` as follows::

   % ls
   phonopy_disp.yaml  POSCAR  POSCAR-001  POSCAR-002  POSCAR-003  SPOSCAR

``SPOSCAR`` is the perfect supercell structure, ``phonopy_disp.yaml``
contains the information on displacements, and ``POSCAR-{number}`` are
the supercells with atomic displacements. ``POSCAR-{number}``
corresponds to the different atomic displacements written in
``phonopy_disp.yaml``.

Calculation of sets of forces
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Force constants are calculated using the structure files
``POSCAR-{number}`` (from forces on atoms) or using the ``SPOSCAR``
file. In the case of VASP, the calculations for the finite
displacement method can be proceeded just using the
``POSCAR-{number}`` files as ``POSCAR`` of VASP calculations. An
example of the ``INCAR`` is as follows::

      PREC = Accurate
    IBRION = -1
     ENCUT = 500
     EDIFF = 1.0e-08
    ISMEAR = 0; SIGMA = 0.01
     IALGO = 38
     LREAL = .FALSE.
     LWAVE = .FALSE.
    LCHARG = .FALSE.

Be careful not to relax the structures. Then create ``FORCE_SETS``
file using :ref:`vasp_force_sets_option`::

   % phonopy -f disp-001/vasprun.xml disp-002/vasprun.xml disp-003/vasprun.xml

or

::

   % phonopy -f disp-{001..003}/vasprun.xml

If you want to calculate force constants by VASP-DFPT directory, see
:ref:`vasp_dfpt_interface`.

Post-process
~~~~~~~~~~~~~

In the post-process,

1. Force constants are calculated from the sets of forces
2. A part of dynamical matrix is built from the force constants
3. Phonon frequencies and eigenvectors are calculated from the
   dynamical matrices with the specified *q*-points.

For mesh sampling calculation, prepare the following setting file
named, e.g., ``mesh.conf``::

   ATOM_NAME = Si O
   DIM = 2 2 3
   MP = 8 8 8

The density of states (DOS) is plotted by::

   % phonopy -p mesh.conf

Thermal properties are calculated with the sampling mesh by::

   % phonopy -t mesh.conf

You should check the convergence with respect to the mesh numbers.
Thermal properties can be plotted by::

   % phonopy -t -p mesh.conf

Projected DOS is calculated by the following setting file named, e.g.,
``pdos.conf``::

   ATOM_NAME = Si O
   DIM = 2 2 3
   MP = 8 8 8
   PDOS = 1 2, 3 4 5 6

and plotted by::

   % phonopy -p pdos.conf

Band structure is calculated with the following setting file named,
e.g., `band.conf` by::

   ATOM_NAME = Si O
   DIM =  2 2 3
   BAND = 0.5 0.5 0.5  0.0 0.0 0.0  0.5 0.5 0.0  0.0 0.5 0.0

The band structure is plotted by::

   % phonopy -p band.conf

In either case, by setting the ``-s`` option, the plot is going to be
saved in the PDF format. If you don't need to plot DOS, the (partial)
DOS is just calculated using the ``--dos`` option.

Non-analytical term correction (Optional)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To activate non-analytical term correction, :ref:`born_file` is
required. This file contains the information of Born effective charge
and dielectric constant. These physical values are also obtained from
the first-principles calculations, e.g., by using VASP, pwscf, etc. In
the case of VASP, an example of ``INCAR`` will be as shown below::

       PREC = Accurate
     IBRION = -1
     NELMIN = 5
      ENCUT = 500
      EDIFF = 1.000000e-08
     ISMEAR = 0
      SIGMA = 1.000000e-02
      IALGO = 38
      LREAL = .FALSE.
      LWAVE = .FALSE.
     LCHARG = .FALSE.
   LEPSILON = .TRUE.

In addition, it is recommended to increase the number of k-points to
be sampled. Twice the number for each axis may be a choice. After
running this VASP calculation, ``BORN`` file has to be created
following the ``BORN`` format (:ref:`born_file`). However for VASP, an
auxiliary tool is prepared, which is ``phonopy-vasp-born``. There is
an option ``--pa`` for this command to set a transformation matrix
from supercell or unit cell with centring to the primitive cell. Since
this rutile-type SiO2 has the primitive lattice, it is unnecessary to
set this option. Running ``phonopy-vasp-born`` in the directory
containing ``vasprun.xml`` (or ``OUTCAR``) of this VASP calculation::

   % phonopy-vasp-born
   # epsilon and Z* of atoms 1 3
      3.2605670   0.0000000   0.0000000   0.0000000   3.2605670   0.0000000   0.0000000   0.0000000   3.4421330
      3.7558600   0.3020100   0.0000000   0.3020100   3.7558600   0.0000000   0.0000000   0.0000000   3.9965200
     -1.8783900  -0.5270900   0.0000000  -0.5270900  -1.8783900   0.0000000   0.0000100   0.0000100  -1.9987900

To employ symmetry constraints, ``--st`` option may used as follows::

   % phonopy-vasp-born --st
   # epsilon and Z* of atoms 1 3
      3.2605670   0.0000000   0.0000000   0.0000000   3.2605670   0.0000000   0.0000000   0.0000000   3.4421330
      3.7561900   0.3020100   0.0000000   0.3020100   3.7561900   0.0000000   0.0000000   0.0000000   3.9968733
     -1.8780950  -0.5270900   0.0000000  -0.5270900  -1.8780950   0.0000000   0.0000000   0.0000000  -1.9984367

The values are slightly modified by symmetry, but we can see the
original values obtained directly from VASP was already very good.

To put ``BORN`` file in the current directly, and running phonopy with
``--nac`` option, non-analytical term correction is activated, such as::

   % phonopy -p --nac band.conf

Please watch the example of NaCl with and without ``--nac`` option shown in
:ref:`examples_link`.