Bethe-Salpeter kernel

Prerequisites

Run Initialization

Run Static screening

Run GW in PPA

Create the input

$ yambo -F 03_bse_Ws.in -b -o b -k sex -V qp   -J 3D (for a 3D)

or

$ yambo -F 03_bse_Ws_2D.in -b -o b -k sex -V qp -r  -J 2D (for a 2D)

Open it and change some of the input variables First change BSEBands which specify the range of bands to be used in the construction of the BS kernel Remember that the BS kernel is written in Bloch space and its size is given by

BS kernel size = Valence Bands × Conduction Bands × K-points in the whole BZ

For h-BN put:

 % BSEBands
 4 |  12 |                   # [BSK] Bands range

For h-BN 2D put:

 % BSEBands
 2 |  6 |                   # [BSK] Bands range

Then change the number of G-components (in number of RL or in energy cutoff) for the exchange(BSENGexx) and correlation (BSENGBlk) part of the BS kernel Both for h-BN or h-BN 2D put

 BSENGexx=  30 Ry    # [BSK] Exchange components
 BSENGBlk=  2 Ry    # [BSK] Screened interaction block size

The GW energies are required in the excitonic hamiltonian. If the rigid scissor assumption is a good approximation for the material of interest the simplest way is to give the value of the minimum GW correction precalculated by you or found in the literature.

For hBN bulk:

 % KfnQP_E
   value0000 | 1.000000 | 1.000000 |        # [EXTQP BSK BSS] E parameters  (c/v) eV|adim|adim

For hBN-2D:

 % KfnQP_E
   value | 1.000000 | 1.000000 |        # [EXTQP BSK BSS] E parameters  (c/v) eV|adim|adim

Note that if you know also the values of the conduction and valence streachings you can insert them instead of using the default values of 1.

Instead if you have already generated a ndb.QP database for the full set of k-points and energies of the excitonic hamiltonian, you can read it in this way

for hBN bulk:

KfnQPdb= " E < ./3D/ndb.QP "              # [EXTQP BSK BSS] Database

for hBN-2D:

KfnQPdb= " E < ./2D/ndb.QP "              # [EXTQP BSK BSS] Database

Close the input and run the code

$ yambo -F 03_bse_Ws.in -J 3D ( for 3D)

or

$ yambo -F 03_bse_Ws_2D.in -J 2D (for 2D)