Si C

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 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.