SiC
===

Author: Samuel Poncé

.. highlight:: none

.. warning::
    The following example aims at providing physically meaningful results. Calculations can therefore take a significant amount of time. For quick calculations, look at the ``EPW/tests/`` folder.

Running EPW on bulk SiC
-----------------------

The bulk SiC example is located inside the ``EPW/examples/sic/`` directory.  Within this directory there are three directories. ``pp/`` contains the Pb pseudopotential, ``phonon/`` contains the input files to calculate the phonons for SiC; ``epw/`` contains the inputs to run ``epw.x`` on SiC.

Once ``pw.x``, ``ph.x`` and ``epw.x`` have been compiled, we are ready to run the example.

The first step is to calculate the phonons in the irreducible wedge.  For this example, we use a ``6x6x6`` coarse grid. 
First go inside the phonon directory 

::

    cd phonons

The phonon code from QE requires a ground-state self-consistent run

::

    ../../../../bin/pw.x < scf.in >& scf.out &

Calculating the phonons
-----------------------

We will now continue in sequential. Let us compute the dynamical matrix, phonon frequencies and change of potential using the ``ph.x`` code

::

    ../../../../bin/ph.x < ph.in >& ph.out &

This will give us the phonon frequency and dynamical matrices at 16 irreducibles q-points.
We then need to copy the ``.dyn`` and ``.dvscf`` as well as the ``_ph0/diam.phsave`` folder inside the ``save`` folder. Those are all the quantities produced by Quantum Espresso that EPW needs.
Because the files need to have a specific name and because there can be quite a few files, you can use the small python script to help you. Just issue

::

    python pp.py

in the phonons folder. The script will ask you the prefix used for the QE calculations as well as the number of irreducible **q**-points computed. The script will place all the files in the ``save`` folder. We are now done with QE and can move to the ``epw`` folder.

Calculating the phonon linewidth using EPW
------------------------------------------

We first have to do a scf and nscf calculations. To do that, go inside the ``epw`` directory and issue:

::

    ../../../../bin/pw.x < scf.in >& scf.out 

::

    ../../../../bin/pw.x < nscf.in >& nscf.out 

We can then run the EPW calculation. We will start by computing the phonon linewidth (imaginary part of the phonon self-energy). Note that we have set ``phonselfen  = .true.`` in the ``epw.in`` input file. 

You can then launch the EPW calculation:

::

    ../../../bin/epw.x < epw.in >& epw.out 

This run will create a number of files like ``linewidth.phself`` for the phonon linewidth, the ``a2f.01`` file for the Eliashberg spectral function.