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# Plotting the BSE fatband structure of Si

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Overview > Dielectric properties of Si using BSE > Improving the dielectric function > Plotting the BSE fatband structure of Si > List of tutorials

## Contents

## Task

Visualization of BSE eigenvectors using fatbands.

## Input

Si 5.4300 0.5 0.5 0.0 0.0 0.5 0.5 0.5 0.0 0.5 2 cart 0.00 0.00 0.00 0.25 0.25 0.25

### INCAR

- This is the INCAR file for the basic DFT calculation:

System = Si PREC = Normal ; ENCUT = 250.0 ISMEAR = 0 ; SIGMA = 0.01 KPAR = 2 EDIFF = 1.E-8 NBANDS = 16 LOPTICS = .TRUE. # needed for WAVEDER file LPEAD = .TRUE. OMEGAMAX = 40

### KPOINTS

The KPOINTS file will be automatically generated in this example.

## Calculation

- In this example we will calculate and plot the first "bright" BSE eigenstates of silicon, also called fatbands
^{[1]}. In this tutorial the modelBSE setup from the previous tutorial is used in combination with a gamma centered k-point grid (if you don't want to wait that long, a grid takes only a minute). In principle the standard BSE method can also be used instead.

### Step 1 DFT calculation

We run a standard DFT calculation with the input files given above.

### Step 2 BSE calculation with fatbands

- The INCAR file for the modelBSE calculation looks like the following:

System = Si PREC = Normal ; ENCUT = 250.0 #ALGO = BSE ANTIRES = 0 ISMEAR = 0 ; SIGMA = 0.01 ENCUTGW = 150 EDIFF = 1.E-8 # default 1.E-4 NBANDS = 16 # only bands that are used are required, prepare the same set in a forgoing DFT run NBANDSO = 4 NBANDSV = 8 OMEGAMAX = 20 PRECFOCK = Normal ALGO = TDHF LMODELHF = .TRUE. #Turn model on HFSCREEN = 1.26 # Screening lenght AEXX = 0.088 #Inverse of epsilon_infinity SCISSOR = 0.69 # Difference GW-DFT band gap NBSEEIG = 10 # number of BSE eigenvectors written out in BSEFATBAND

- The important tag for fatband calculations is NBSEEIG. In this example this will write the 10 energetically lowest BSE eigenvectors to the output file BSEFATBAND.
- After the (model)BSE calculation we first look into the vasprun.xml file to have a look at the BSE eigenvalues and the oscillator strengths:

vasprun.xml (exact numbers can of course differ depending on the method you use and the density of your k-point grid):

<varray name="opticaltransitions" > <v> 3.148 0.000 </v> <v> 3.148 0.000 </v> <v> 3.148 0.000 </v> <v> 3.149 25654.244 </v> <v> 3.149 25660.601 </v> <v> 3.149 25665.652 </v> <v> 3.151 0.001 </v> <v> 3.151 0.001 </v> <v> 3.152 423.751 </v> <v> 3.310 216931.963 </v> <v> 3.310 216916.814 </v> <v> 3.310 216935.593 </v>

- The first number column shows the BSE eigenvalue and the second one the oscillator strength. We want to plot the first "bright" state, that means in this case number 4.

## Download

## References

Overview > Dielectric properties of Si using BSE > Improving the dielectric function > Plotting the BSE fatband structure of Si > List of tutorials

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