# Calculating optical spectra including excitonic effects: a step-by-step guide

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.

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 ε_{M}. The following quantities/steps are needed to obtain ε_{M}:

The optical absorption spectrum corresponds to Imε_{M}(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 type:

$ 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. In this tutorial, we diagonalise the whole Bethe-Salpeter matrix, but there are other numerical approaches available in Yambo. The difference between these 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 **return to this tutorial**

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

In Step 3, we included the quasiparticle corrections to the Kohn-sham energies as a scissor operator. An alternative is to use the results from a previous run of Yambo.

Follow the module on **Bethe-Salpeter on top of quasiparticle energies** and **return to this tutorial**

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