.. _castep_interface:

CASTEP & phonopy calculation
============================

Examples to use phonopy with CASTEP as the force calculator are found
at ``example/NaCl-castep`` and ``example/Cr-castep``.

The followings are the copies of ``README`` contents in these example
directories.

These are contriubted by `@ladyteam <https://github.com/ladyteam>`_.


NaCl-castep
-----------

This is step-by-step example how to calculate phonon dispertion with
CASTEP.

1. Generate supercell based on structrural parameters given in
   ``unitcell.cell`` file by running::

      % phonopy -d --dim="2 2 2" --castep -c unitcell.cell

   In this example file ``unitcell.cell`` contains initial spin values in
   ``positions_frac`` BLOCK. As well as the value of the U parameter in
   LDA+U approximation.

2. Use ``make_displ_dirs.sh`` bash script to make directories with input
   files needed to calculate FORCES for supercells.

   ::

      % make_displ_dirs.sh supercell.param tail.cell

   Please note file supercell.param is the standart CASTEP file with
   paremeters (convergence criteria, Ecut, functional name etc.). The
   file with name ``tail.cell`` contains k-mesh description, U parameter
   value, symmetry operation E, and name of pseudopotential.

3. Run CASTEP calculation in ``displ-001`` directory

4. Generate ``FORCE_SETS`` file by running::

      % phonopy --castep -f displ-001/supercell.castep displ-002/supercell.castep

5. Finally plot phonon bandstructure by running the following command::

      % phonopy --castep -c unitcell.cell -p band.conf


Cr-castep
---------

This is step-by-step example how to calculate phonon dispertion with CASTEP.
Spin-polarization example.

1. Generate supercell based on structrural parameters given in
   ``unitcell.cell`` file by running::

      % phonopy -d --dim="2 2 2" --castep -c unitcell.cell

   In this example file ``unitcell.cell`` contains initial spin values in
   ``positions_frac`` BLOCK. As well as the value of the U parameter in
   LDA+U approximation.

2. Use ``make_displ_dirs.sh`` bash script to make directories with input
   files needed to calculate FORCES for supercells.

   ::

      % make_displ_dirs.sh supercell.param tail.cell

   Please note file ``supercell.param`` is the standart CASTEP file with
   paremeters (convergence criteria, Ecut, functional name etc.). The
   file with name ``tail.cell`` contains k-mesh description, U parameter
   value, symmetry operation E, and name of pseudopotential.

3. Run CASTEP calculation in ``displ-001`` directory

4. Generate ``FORCE_SETS`` file by running::

      % phonopy --castep -f displ-001/supercell.castep

5. Finally plot phonon bandstructure by running the following command:

   ::

      % phonopy --dim="2 2 2" --pa="-1/2 1/2 1/2 1/2 -1/2 1/2 1/2 1/2 -1/2" --castep -c unitcell.cell --band="1/2 -1/2 1/2 0 0 0 1/4 1/4 1/4" -p
