Dielectric properties of Si using BSE: Difference between revisions

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*The workflow of GW0+BSE calculations is given in doall.sh and consists of the following consecutive steps:
*The workflow of GW0+BSE calculations is given in doall.sh and consists of the following consecutive steps:
# "Standard" DFT ground state calculation.
# "Standard" DFT groundstate calculation.
# Obtain virtual orbitals: needs {{TAG|WAVECAR}} file from step 1.
# Obtain virtual orbitals: needs {{TAG|WAVECAR}} file from step 1.
# The GW0 calculation: need {{TAG|WAVECAR}} and {{TAG|WAVEDER}} from step 2.
# The GW0 calculation: need {{TAG|WAVECAR}} and {{TAG|WAVEDER}} from step 2.
Line 42: Line 42:
# The BSE calculation: needs {{TAG|WAVECAR}} from step 3 and {{TAG|WAVEDER}} from step 2.
# The BSE calculation: needs {{TAG|WAVECAR}} from step 3 and {{TAG|WAVEDER}} from step 2.


   
=== Step 1: DFT groundstate calculation ===
*We perform standard DFT calculation using the input parameters from above.
 
=== Step 2: Obtain DFT "virtual" orbitals (empty states) ===
*This step uses the following {{TAG|INCAR}} file:
  {{TAGBL|System}}  = Si
{{TAGBL|PREC}} = Normal ; {{TAGBL|ENCUT}} = 250.0
{{TAGBL|ALGO}} = EXACT ; {{TAGBL|NELM}} = 1
{{TAGBL|ISMEAR}} = 0 ; {{TAGBL|SIGMA}} = 0.01
{{TAGBL|KPAR}} = 2
{{TAGBL|NBANDS}} = 128
{{TAGBL|LOPTICS}} = .TRUE. ; {{TAGBL|LPEAD}} = .TRUE.
{{TAGBL|OMEGAMAX}} = 40
*We use exact diagonalization for this step ({{TAG|ALGO}}=''EXACT'') and keep 128 bands after diagonalization ({{TAG|NBANDS}}=128).
*With {{TAG|LPEAD}}=''.TRUE.'' we use an alternative way of computing the derivates of the orbitals with respect to the Bloch wave vectors.
*It is important that this calculations needs the orbitals ({{TAG|WAVECAR}} file) written in step 1.
 
=== Step3 3: RPA quasiparticles with single-shot GW (G0W0) ===
*This step uses the following {{TAG|INCAR}} file:
{{TAGBL|System}}  = Si
{{TAGBL|PREC}} = Normal ; {{TAGBL|ENCUT}} = 250.0
{{TAGBL|ALGO}} = GW0 
{{TAGBL|ISMEAR}} = 0 ; {{TAGBL|SIGMA}} = 0.01
{{TAGBL|ENCUTGW}} = 150 ; {{TAGBL|NELM}} = 1 ;  {{TAGBL|NOMEGA}} =  50 ;  {{TAGBL|OMEGATL}} = 280
{{TAGBL|KPAR}} = 2
#{{TAGBL|NBANDSO}}=4 ; {{TAGBL|NBANDSV}}=8 ; {{TAGBL|LADDER}}=.TRUE. ; {{TAGBL|LUSEW}}=.TRUE.
{{TAGBL|NBANDS}} = 128
{{TAGBL|NBANDSGW}} = 12
{{TAGBL|LWAVE}} = .TRUE.
{{TAGBL|PRECFOCK}} = Normal
*We select the G0W0 method by specifying {{TAG|ALGO}}=''GW0'' and {TAG|NELM}}=1.
*The energy cut off for the response function is select by {{TAG|ENCUTGW}}.
*The number of point used in the frequency integration is given by {{TAG|NOMEGA}}.
*Use the same number of bands ({{TAG|NBANDS}}) as in step 2, otherwise the {{TAG|WAVEDER}} file is not read correctly.
*The quasiparticle energies are calculated for the first few bands given by {{TAG|NBANDSGW}}.
*It is important that this calculation needs the orbitals ({{TAG|WAVECAR}} file) and the derivatives of the orbitals with respect to the Bloch vectors ({{TAG|WAVEDER}} file).
*The quasiparticle energies can be found in the {{TAG|OUTCAR}} file (saved as OUTCAR.GW0 in this example):
QP shifts <psi_nk| G(iteration)W_0 |psi_nk>: iteration 1
for sc-GW calculations column KS-energies equals QP-energies in previous step
and V_xc(KS)=  KS-energies - (<T + V_ion + V_H > + <T+V_H+V_ion>^1  + <V_x>^1)
k-point  1 :      0.0000    0.0000    0.0000
band No.  KS-energies  QP-energies  sigma(KS)  V_xc(KS)    V^pw_x(r,r')  Z            occupation


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Revision as of 17:02, 10 November 2017

Task

Description: Calculate the dielectric function of Si including excitonic effects by solving the Bethe-Salpeter equation (BSE) on top of GW0.

Input

POSCAR

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

KPOINTS

Automatic
 0
Gamma
 6 6 6 
 0 0 0

Calculation

  • The workflow of GW0+BSE calculations is given in doall.sh and consists of the following consecutive steps:
  1. "Standard" DFT groundstate calculation.
  2. Obtain virtual orbitals: needs WAVECAR file from step 1.
  3. The GW0 calculation: need WAVECAR and WAVEDER from step 2.
  4. Optional step: use LOPTICS=.TRUE. to plot dielectric function in the independent particle approximation (IPA) using GW0 quasiparticle energies instead of DFT energies.
  5. The BSE calculation: needs WAVECAR from step 3 and WAVEDER from step 2.

Step 1: DFT groundstate calculation

  • We perform standard DFT calculation using the input parameters from above.

Step 2: Obtain DFT "virtual" orbitals (empty states)

  • This step uses the following INCAR file:
System  = Si
PREC = Normal ; ENCUT = 250.0
ALGO = EXACT ; NELM = 1
ISMEAR = 0 ; SIGMA = 0.01
KPAR = 2
NBANDS = 128
LOPTICS = .TRUE. ; LPEAD = .TRUE.
OMEGAMAX = 40
  • We use exact diagonalization for this step (ALGO=EXACT) and keep 128 bands after diagonalization (NBANDS=128).
  • With LPEAD=.TRUE. we use an alternative way of computing the derivates of the orbitals with respect to the Bloch wave vectors.
  • It is important that this calculations needs the orbitals (WAVECAR file) written in step 1.

Step3 3: RPA quasiparticles with single-shot GW (G0W0)

  • This step uses the following INCAR file:
System  = Si
PREC = Normal ; ENCUT = 250.0
ALGO = GW0  
ISMEAR = 0 ; SIGMA = 0.01 
ENCUTGW = 150 ; NELM = 1 ;  NOMEGA =  50 ;  OMEGATL = 280
KPAR = 2
#NBANDSO=4 ; NBANDSV=8 ; LADDER=.TRUE. ; LUSEW=.TRUE.
NBANDS = 128
NBANDSGW = 12
LWAVE = .TRUE.
PRECFOCK = Normal
  • We select the G0W0 method by specifying ALGO=GW0 and {TAG|NELM}}=1.
  • The energy cut off for the response function is select by ENCUTGW.
  • The number of point used in the frequency integration is given by NOMEGA.
  • Use the same number of bands (NBANDS) as in step 2, otherwise the WAVEDER file is not read correctly.
  • The quasiparticle energies are calculated for the first few bands given by NBANDSGW.
  • It is important that this calculation needs the orbitals (WAVECAR file) and the derivatives of the orbitals with respect to the Bloch vectors (WAVEDER file).
  • The quasiparticle energies can be found in the OUTCAR file (saved as OUTCAR.GW0 in this example):
QP shifts <psi_nk| G(iteration)W_0 |psi_nk>: iteration 1
for sc-GW calculations column KS-energies equals QP-energies in previous step
and V_xc(KS)=  KS-energies - (<T + V_ion + V_H > + <T+V_H+V_ion>^1  + <V_x>^1)
k-point   1 :       0.0000    0.0000    0.0000
band No.  KS-energies  QP-energies   sigma(KS)   V_xc(KS)     V^pw_x(r,r')   Z            occupation 

Used INCAR Tags

ALGO, ANTIRES, EDIFF, ENCUT, ENCUTGW, IALGO, IMIX, ISMEAR, KPAR, LOPTICS, LPEAD, LWAVE, NBANDS, NBANDSGW, NBANDSO, NBANDSV, NELM, NKRED, NOMEGA, OMEGAMAX, OMEGATL, PREC, PRECFOCK, SIGMA

Download

Si_BSE.tgz

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