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Plotting the BSE fatband structure of Si
Visualization of BSE eigenvectors using fatbands.
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
- 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
The KPOINTS file will be automatically generated in this example.
- 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 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.
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