Bethe-Salpeter

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In this tutorial

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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 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 create the appropriate input: 1) yambo -b This will create the input to calculate W (database ndb1.em1s) 2) yambo -p p This will create the input to calculate W in the PPA approximation (database ndb.pp) This second option is useful if you already have created a database ndb.pp in a previous GW run.

At the end of the run you will find a new database ndb.em1s. It will be in ./SAVE direcory if you did not use the -J option

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