Electron Phonon Coupling

Here we show step-by-step how to use Quantum Espresso to calculate phonons and electron-phonon matrix-elements on a regular q-grid, with the final aim to allow Yambo to read these databases and calculate the temperature-dependent correction to the electronic states. This tutorial is quite complicated, take your time to follow all the steps

Calculations will be divided in different folders:

• pseudo the pseudo potential folder
• scf for the self-consistent calculation
• nscf for the non-self-consistent calcaultion with a larger number of bands
• phonon for the phonons calculations
• dvscf for the calculation of electron-phonon matrix elements

In this tutorial we will show how to calculate electron-phonon induced corrections to the bands and optical properties of 2D hexagonal boron nitride. All input file are availabe in the following tgz file: hBN.epc.tgz

1. In scf we run a standard scf calculation choosing the a large k-grid in such a way to converge density. Do not forget to set force_symmorphic=.true., because not symmorphic symmetries are not supported yet in Yambo. Notice that because the present system is two-dimensional we added the flag assume_isolated="2D" in such a way correct phonons in 2D, remove this flag is you have a system with a different dimensionality (a bulk, a molecule etc...)

2. Go in the nscf folder, and then copy the ${PREFIX}.save folder from scf to nscf, in the present example just do cp -r ../scf/bn.save ./. In the nscf input you have to choose the number of k-points and bands you will use for the electron-phonon coupling and Yambo calculations, in our case we will a 9x9x1 grid and 8 bands.

3. Go in the phonon directory. You have to copy the k-points list from the previous nscf run and provide it as q-grid for the phonon calculations. Notice that due to an internal convenction of Yambo the q-points have to be taken with the minus sign. Using the following python script xxxxx to extract k-points for QE output and add them the minus sign: read_q.py -F output_nscf. Then add them to the phonons input:

&inputph
           verbosity = 'high'
              tr2_ph = 1e-16
              prefix = 'bn'
            fildvscf = 'bn-dvscf'
              fildyn = 'bn.dyn'
     electron_phonon = 'dvscf',
               epsil = .true.
               trans = .true.
               ldisp = .false.
               qplot = .true.
/

&inputph
              tr2_ph = 1e-16
              prefix = '6HSiC'
            fildvscf = '6HSiC-dvscf'
              fildyn = '6HSiC.dyn'
     electron_phonon = 'dvscf',
               epsil = .true.
               trans = .true.
               ldisp = .true.
           verbosity = 'high'
         nq1=10, nq2 =10, nq3=2
/

3. In nscf folder I run an nscf calculation, setting the number of bands nbnd equal to the desired band number, force_symmorphic=.true. and the same q grid as before. A ${PREFIX}.save folder will be automatically created.

4. In the main directory I copy and then overwrite the previous ${PREFIX}.save directory with the new one. Now I run an elph calculation setting electron_phonon = ‘yambo’, and the q grid.

&inputph
    fildvscf = '6HSiC-dvscf'
    fildyn = '6HSiC.dyn'
           verbosity = 'high'
               epsil = .true.
               ldisp = .true.
              tr2_ph = 1e-16
              prefix = '6HSiC'
     electron_phonon = 'yambo',
               trans = .false.
         nq1=10, nq2 =10, nq3=2
/