Bethe-Salpeter

UNDER CONSTRUCTION (MP)

In this tutorial you will learn how to:

• generate input files to run a BSE calculation for a two-dimensional material
• run the code and control inputs and outputs
• understand the content of the outputs

Prerequisites

File:HBN-2D.png

Atomic structure of two-dimensional hBN

Material: two-dimensional hexagonal BN:

• exagonal lattice
• 2 atoms per cell, B and N (8 electrons)

Atomic structure of bulk hBN

• Lattice constants: a = 4.716 [a.u.]
• Plane wave cutoff 40 Ry

To do the Tutorial you have two options:

  Option A): Start from the beginning (DFT runs also)
  Option B): Read directly a prepared SAVE database 

If you choose Option B) look below for Option B): Read directly a prepared SAVE database and start from there. Please note that in this case you do not need to run the initialization, cutoff generation, calculation of screening W and quasi-particle energies. All the databases ndb.gops ndb,kindx ndb.RIM ndb.cutoff ndb.pp ndb.QP have been already generated for you and are in the SAVE directory

Option A: Start from the beginning (DFT runs also)

In this case you will need:

• PWSCF input files and pseudopotentials for hBN-2D
• pw.x executable, version 5.0 or later
• p2y and yambo executables

Unpack the TARFILE:

$ tar -xcvf hBN-2D.tar

Go in the PWSCF directory and list the files.

$ cd hBN-2D/PWSCF
$ ls
hbn-2D_scf.in hbn-2D_nscf.in References tmp

Have a look at the main variables in the inputs hbn-2D_scf.in hbn-2D_nscf.in: k-points, nbnd etc.

Run the DFT self-consistent (scf) and non-self-consistent (nscf) inputs.

$ pw.x < ./Inputs/hbn_2d_scf.in > hbn_2d_scf.out
$ pw.x < ./Inputs/hbn_2d_scf.in > hbn_2d_nscf.out

Generate the SAVE database

$ cd ./tmp/hbn_2d.save
$ p2y 

Move the SAVE database in the YAMBO directory to start the calculations with yambo

$ mv  SAVE ../../../YAMBO
$ cd ../../../YAMBO
$ ls
SAVE

Initialization

As you should know any Yambo run must start with the Initialization step. Simply launch the code

 $ yambo 

This will run the initialization (setup) runlevel.
TIP: do not run yambo from inside the SAVE folder! It will complain that "databases not found".

Option B): Read directly a prepared SAVE database

Note that in this case in the SAVE directory you have already the following databases ndb.gops ndb.kindx ndb.RIM ndb.cutoff ndb.pp ndb.QP databases

$ tar -xcvf hBN-2D.tar $ cd hBN-2D/YAMBO $ mv ./References/SAVE SAVE

Initialization

Every Yambo run must start with this step. Go to the folder containing the hBN-2D SAVE directory:

$ cd TUTORIALS/hBN-2D/YAMBO
$ ls
SAVE

and simply launch the code

yambo 

This will run the initialization (setup) runlevel.
TIP: do not run yambo from inside the SAVE folder! It will complain that "databases not found".

BSE Tutorials at present:

• SiH_4

• Fantastic dimension

• LiF

• you Hydrogen

• GaSb (SOC)

• hexagonal BN (on slides only)

The material

The h-BN is a layered material similar to graphite. It is an indirect wide band-gap material. Its optical spectrum is characterized by large excitonic effects. 5.95 eV is the minimum electronic indirect gap. 6.47 eV is the minimum direct gap. See the literature [1]

Bulk hexagonal BN [2] hexagonal lattice Two atoms per cell (16 electrons) Lattice parameters a = 4.72 [a.u.] c/a= 2.58 Plane waves cutoff 40 Rydberg

[[File:|Band Structure]]

Calculate the screening

A key ingredient to construct the BS kernel in the Screeened EXchange approximation (sex) is the screened electron-electron dielectric matrix W which is normally evaluated in the static approximation.

You have two main options to generate the input :

 1) yambo -b -F  yambo_Ws.in 

This will create the input to calculate W (database ndb.em1s)

 2) yambo -p p -F  yambo_Wp.in 

This will create the input to calculate (or read) W in the PPA approximation (database ndb.pp) This second option is useful if you want to read the static part of W from a previous database ndb.pp generated in a GW-PPA run

Calculate the BS kernel in the SEX (Screened Exchange approximation) in the transitions space

Here we learn how to create the BS kernel in the screened exchange (SEX) approximation which includes both exchange (V) and correlation (-W) terms Runlevel to be used yambo -o b -k sex

Solve the BSE by haydock solver and and calculate spectrum

Generate the input file for solving the BSE using the Haydock solver:

yambo  -o-y h

Solve the BSE by diagonalizing the excitonic matrix and calculate spectrum

Runlevel to be used yambo -y d

Steps:

-Calculate screening

-Calculate the BS Kernel

-Diagonalize the BS Matrix and calculate spectrum

-Visualize/Analyze excitons (with ypp)

-How to Converge

-How to work with SOC