## Ga N

Author: Samuel Poncé

⚠ 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 polar wurtzite GaN**

The polar wurtzite GaN example is located inside the `EPW/examples/gan/`

directory. Within this directory there are three directories. `pp/`

contains the Pb pseudopotential, `phonon/`

contains the input files to calculate the phonons for GaN; `epw/`

contains the inputs to run `epw.x`

on GaN.

Once `pw.x`

, `ph.x`

and `epw.x`

have been compiled, we are ready to run the example.

**Calculating the phonons**

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 &

The electronic bandstructure of w-GaN is presented in Figure 1 below.

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 28 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.

The phonon bandstructure of w-GaN is presented in Figure 1 below.

**Calculating the electron-phonon matrix element 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.
Also you can notice that we are computing the interpolated electron-phonon matrix element along the high-symmetry **q**-point line. This is achieved by reading a data file using the `filqf = 'gan_band.qpt'`

input command.

You can then launch the EPW calculation:

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

Although the electron-phonon matrix elements as presented at the bottom of Figure 1 are not outputted directly (usually not relevant), you can easily print them by modifying the code. This should give you the correct red interpolation line as seen on the figure.