Plotting the BSE fatband structure of Si

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

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. In this tutorial the modelBSE setup from the previous tutorial is used in combination with a $10 \times 10 \times 10$ gamma centered k-point grid (if you don't want to wait that long, a $4 \times 4 \times 4$ 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

Si_BSEfatbands.tgz

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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