Difference between revisions of "Calculating optical spectra including excitonic effects: a step-by-step guide"

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===Step 3: Diagonalisation of the excitonic Hamiltonian ===
===Step 3: Diagonalisation of the excitonic Hamiltonian ===
This is the final step in which you finally obtained the spectra.
This is the step in which you obtain the spectra. Mathematically this implies solving a large eigenvalue problem. To this scope, there are different numerical approaches in Yambo. The difference between the various approaches and when they should be used is the object of one of the next tutorials.  
Mathematically this implies to solve a large eigenvalue problem.  


'''Part that should be moved to solvers tutorial'''
Follow the module on '''[[Bethe-Salpeter solver: diagonalization]]''' then either  '''return to this tutorial'''  


Two main solvers are available in yambo
(1) diagonalization of the full Hamiltonian (diagonalization solver)
(2) subspace iterative [https://en.wikipedia.org/wiki/Lanczos_algorithm | Lanczos algorithm] which by-pass diagonalization with the Haydock approach<ref>R. Haydock, in
''Solid State Phys.'', '''35''' 215 (1980) edited by H. Ehrenfest, F. Seitz, and D. Turnbull, Academic Press</ref> (Lanczos-Haydock solver)
For (1) follow the module on '''[[Bethe-Salpeter solver: diagonalization]]''' then either  '''return to this tutorial'''
or follow the link to '''[[Bethe-Salpeter solver: Lanczos-Haydock]]''' for (2).
'''End part that should be moved to solvers tutorial '''


===Step 4: Include previous quasiparticle (GW) results ===
===Step 4: Include previous quasiparticle (GW) results ===

Revision as of 15:59, 23 March 2021

This tutorial guides you through the workflow of a calculation of the optical spectrum of a given material by solving the Bethe-Salpeter equation. Specifically we will use bulk h-BN as an example.

Atomic structure of bulk hBN

Before starting, you need to obtain the tarballs for hBN. See instructions on the main tutorials page.

The target quantity in a Bethe-Salpeter calculation is the macroscopic dielectric matrix εM. The following quantities/steps are needed to obtain εM:

BSE calculation scheme

The optical absorption spectrum corresponds to ImεM(ω). Following this scheme we go through the flow of a calculation:

Step 1: Static screening

Use the SAVE folders that are already provided and do: 

$ cd YAMBO_TUTORIALS/hBN/YAMBO

Follow the Static screening module and then return to this tutorial

Step 2: Bethe-Salpeter kernel

Follow the module on Bethe-Salpeter kernel and return to this tutorial

Step 3: Diagonalisation of the excitonic Hamiltonian

This is the step in which you obtain the spectra. Mathematically this implies solving a large eigenvalue problem. To this scope, there are different numerical approaches in Yambo. The difference between the various approaches and when they should be used is the object of one of the next tutorials.

Follow the module on Bethe-Salpeter solver: diagonalization then either return to this tutorial


Step 4: Include previous quasiparticle (GW) results

Follow the module on Including quasiparticle database and return to this tutorial


References

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