Bandgap of Si using different DFT+HF methods

Based on the VASP wiki example in this link

Task: Calculate the bandgap of Si using different DFT+HF schemes: PBE, B3LYP, PBE0, HSE06 and HF.

First, copy the example folder which contains some of the VASP input files and useful scripts

cp -r /software/sse/manual/vasp/training/ws2020/bandgap_Si .
cd bandgap_Si

and copy the latest POTCAR file for Si

cp /software/sse/manual/vasp/POTCARs/PBE/2015-09-21/Si/POTCAR .

Input files

POSCAR

System: Si                             
 5.430 
 0.5 0.5 0.0
 0.0 0.5 0.5
 0.5 0.0 0.5
Si
   1  
Cartesian
0    0    0
0.25 0.25 0.25

INCAR

## Better preconverge with PBE first
## and use the WAVECAR file as inout for the DFT+HF calculation
   
## Selects the B3LYP hybrid function
#LHFCALC = .TRUE. ; GGA = B3 ; AEXX = 0.2 ; AGGAX = 0.72 
#AGGAC = 0.81 ; ALDAC = 0.19
#ALGO = D ; TIME = 0.4 
   
## Selects the PBE0  hybrid function
#LHFCALC = .TRUE. ; 
#ALGO = D ; TIME = 0.4 
   
## Selects the HSE06 hybrid function
#LHFCALC = .TRUE. ; HFSCREEN = 0.2 ; 
#ALGO = D ; TIME = 0.4 
   
## Selects HF 
#LHFCALC = .TRUE. ; AEXX = 1.0 ; ALDAC = 0.0 ; AGGAC = 0
#ALGO = D ; TIME = 0.4 
   
## Leave this in
ISMEAR =  0
SIGMA  =  0.01
GGA    = PE

• This INCAR will run a standard PBE calculation, since all DFT+HF methods are commented out

KPOINTS

k-points
0
Gamma
  6  6  6
  0  0  0

• Gamma (G) k-mesh -> gamma point included by default

Calculations

First, run the regular PBE calculation which will be used as a start for the DFT+HF methods

sbatch run.sh

when it’s finished, cycle through the different DFT+HF methods, don’t forget to copy WAVECAR

mkdir B3LYP
cp INCAR POSCAR KPOINTS POTCAR run.sh gap.sh WAVECAR B3LYP
cd B3LYP

edit INCAR such that the method of interest is uncommented, e.g.

## Selects the B3LYP hybrid function
LHFCALC = .TRUE. ; GGA = B3 ; AEXX = 0.2 ; AGGAX = 0.72 
AGGAC = 0.81 ; ALDAC = 0.19
ALGO = D ; TIME = 0.4 

• Note the use of ALGO=Damped or D, for the DFT+HF methods, different from the default ALGO=Normal.

and run the job

sbatch run.sh

after it has finished, inspect the output

cat slurm*out
cat OSZICAR

to obtain energies for the highest occupied and lowest unoccupied states, run the provided script “gap.sh” in this way

./gap.sh OUTCAR

The bandgap is then given by: bandgap = min(cband) - max(vband).

In the same way, repeat the calculations for PBE0, HSE06 and HF.

• Compare the results with PBE, how big is the difference?
• Why didn’t we explicitly set ISTART=1 in INCAR in order to continue the calculation? Because, since a WAVECAR was present in the folder, the default is set to ISTART=1 instead of 0. Confirm with “grep ISTART OUTCAR”.