the BSE absorption spectrum does not rise slowly from 0

[shan dong]

Dear all,
I have done a PBE+soc+BSE calculation for treating a 2D system with a 0.3 eV scissor correction.But I found that there is a positive peak at -0.27ev where the energy is less than zero, and the BSE absorption spectrum does not rise slowly from 0, nor is it an odd function about 0.

Could anybody please give me some suggestions? Thank you very much!

[myrta gruning]

It seems that the binding energy is larger than the band gap.
Is the dielectric screening well-converged?
Which diagonalisation scheme are you using?

[shan dong]

It seems that the binding energy is larger than the band gap.
Is the dielectric screening well-converged?
Which diagonalisation scheme are you using?

Dear myrta,
The diagonalisation scheme I used was "resonant".

[Daniele Varsano]

Dear shan dong,

"resonant" is not the diagonalization scheme, but the ordering of the response function and this is why you do not have an odd function with respect to zero.
About the fact the binding energy is larger than the gap, you should check if a scissor correction of 0.3 eV is reasonable for your system.
If you post your input/report file we can check if there is something missing in your input file.

Best,
Daniele

[shan dong]

Dear shan dong,

"resonant" is not the diagonalization scheme, but the ordering of the response function and this is why you do not have an odd function with respect to zero.
About the fact the binding energy is larger than the gap, you should check if a scissor correction of 0.3 eV is reasonable for your system.
If you post your input/report file we can check if there is something missing in your input file.

Best,
Daniele

Dear Daniele,
This is my input for BSE.

Code: Select all

em1s                             # [R][Xs] Statically Screened Interaction
rim_cut                          # [R] Coulomb potential
optics                           # [R] Linear Response optical properties
bss                              # [R] BSE solver
bse                              # [R][BSE] Bethe Salpeter Equation.
dipoles                          # [R] Oscillator strenghts (or dipoles)
StdoHash=  40                    # [IO] Live-timing Hashes
Nelectro= 52.00000               # Electrons number
ElecTemp= 0.000000         eV    # Electronic Temperature
BoseTemp=-1.000000         eV    # Bosonic Temperature
OccTresh= 0.100000E-4            # Occupation treshold (metallic bands)
NLogCPUs=0                       # [PARALLEL] Live-timing CPU`s (0 for all)
DBsIOoff= "none"                 # [IO] Space-separated list of DB with NO I/O. DB=(DIP,X,HF,COLLs,J,GF,CARRIERs,OBS,W,SC,BS,ALL)
DBsFRAGpm= "none"                # [IO] Space-separated list of +DB to FRAG and -DB to NOT FRAG. DB=(DIP,X,W,HF,COLLS,K,BS,QINDX,RT,ELP
MEM_tresh=  51200          Kb    # [MEMORY] Treshold on traced memory allocations/deallocations
FFTGvecs= 18139            RL    # [FFT] Plane-waves
#WFbuffIO                      # [IO] Wave-functions buffered I/O
PAR_def_mode= "memory"         # [PARALLEL] Default distribution mode ("balanced"/"memory"/"workload")
#DIP_CPU= "1 1 30 2"                      # [PARALLEL] CPUs for each role
#DIP_ROLEs= "q k c v"                    # [PARALLEL] CPUs roles (k,c,v)
X_and_IO_CPU= "1 1 30 2"                 # [PARALLEL] CPUs for each role
X_and_IO_ROLEs= "q k c v"               # [PARALLEL] CPUs roles (q,g,k,c,v)
X_and_IO_nCPU_LinAlg_INV=4      # [PARALLEL] CPUs for Linear Algebra (if -1 it is automatically set)
BS_CPU= ""                       # [PARALLEL] CPUs for each role
BS_ROLEs= ""                     # [PARALLEL] CPUs roles (k,eh,t)
BS_nCPU_LinAlg_INV=4            # [PARALLEL] CPUs for Linear Algebra (if -1 it is automatically set)
BS_nCPU_LinAlg_DIAGO=4          # [PARALLEL] CPUs for Linear Algebra (if -1 it is automatically set)
NonPDirs= "none"                 # [X/BSS] Non periodic chartesian directions (X,Y,Z,XY...)
% MolPos
 0.000000 | 0.000000 | 0.000000 |        # [X/BSS] Molecule coord in supercell, 0.5 is the middle
%
RandQpts= 2000000                       # [RIM] Number of random q-points in the BZ
RandGvec= 1                RL    # [RIM] Coulomb interaction RS components
#QpgFull                       # [F RIM] Coulomb interaction: Full matrix
% Em1Anys
 0.000000 | 0.000000 | 0.000000 |        # [RIM] X Y Z Static Inverse dielectric matrix
%
IDEm1Ref=0                       # [RIM] Dielectric matrix reference component 1(x)/2(y)/3(z)
CUTGeo= "box Z"                   # [CUT] Coulomb Cutoff geometry: box/cylinder/sphere/ws X/Y/Z/XY..
% CUTBox
 0.000000 | 0.000000 | 29.000000 |        # [CUT] [au] Box sides
%
CUTRadius= 0.000000              # [CUT] [au] Sphere/Cylinder radius
CUTCylLen= 0.000000              # [CUT] [au] Cylinder length
CUTwsGvec= 0.700000              # [CUT] WS cutoff: number of G to be modified
#CUTCol_test                   # [CUT] Perform a cutoff test in R-space
Chimod= "HARTREE"                # [X] IP/Hartree/ALDA/LRC/PF/BSfxc
ChiLinAlgMod= "LIN_SYS"          # [X] inversion/lin_sys,cpu/gpu
BSEmod= "resonant"               # [BSE] resonant/retarded/coupling
BSKmod= "SEX"                    # [BSE] IP/Hartree/HF/ALDA/SEX/BSfxc
Lkind= "Lbar"                    # [BSE] Lbar (default) / full
BSSmod= "d"                      # [BSS] (h)aydock/(d)iagonalization/(s)lepc/(i)nversion/(t)ddft`
% DipBands
   1 | 300 |                         # [DIP] Bands range for dipoles
%
#DipBandsALL                   # [DIP] Compute all bands range, not only valence and conduction
DipApproach= "G-space v"         # [DIP] [G-space v/R-space x/Covariant/Shifted grids]
DipComputed= "R P V"             # [DIP] [default R P V; extra P2 Spin Orb]
#DipPDirect                    # [DIP] Directly compute <v> also when using other approaches for dipoles
ShiftedPaths= ""                 # [DIP] Shifted grids paths (separated by a space)
DbGdQsize= 1.000000              # [X,DbGd][o/o] Percentual of the total DbGd transitions to be used
Gauge= "length"                  # [BSE/X] Gauge (length|velocity)
#AnHall                        # [BSE] Add the anomalous Hall effect to eps if using length gauge
BSENGexx= 116487           RL    # [BSK] Exchange components
#ALLGexx                       # [BSS] Force the use use all RL vectors for the exchange part
BSENGBlk=7                Ry    # [BSK] Screened interaction block size [if -1 uses all the G-vectors of W(q,G,Gp)]
#WehDiag                       # [BSK] diagonal (G-space) the eh interaction
#WehCpl                        # [BSK] eh interaction included also in coupling
KfnQPdb= "none"                  # [EXTQP BSK BSS] Database action
KfnQP_INTERP_NN= 1               # [EXTQP BSK BSS] Interpolation neighbours (NN mode)
KfnQP_INTERP_shells= 20.00000    # [EXTQP BSK BSS] Interpolation shells (BOLTZ mode)
KfnQP_DbGd_INTERP_mode= "NN"     # [EXTQP BSK BSS] Interpolation DbGd mode
% KfnQP_E
 0.297000 | 1.000000 | 1.000000 |        # [EXTQP BSK BSS] E parameters  (c/v) eV|adim|adim
%
KfnQP_Z= ( 1.000000 , 0.000000 )         # [EXTQP BSK BSS] Z factor  (c/v)
KfnQP_Wv_E= 0.000000       eV    # [EXTQP BSK BSS] W Energy reference  (valence)
% KfnQP_Wv
 0.000000 | 0.000000 | 0.000000 |        # [EXTQP BSK BSS] W parameters  (valence) eV| 1|eV^-1
%
KfnQP_Wv_dos= 0.000000     eV    # [EXTQP BSK BSS] W dos pre-factor  (valence)
KfnQP_Wc_E= 0.000000       eV    # [EXTQP BSK BSS] W Energy reference  (conduction)
% KfnQP_Wc
 0.000000 | 0.000000 | 0.000000 |        # [EXTQP BSK BSS] W parameters  (conduction) eV| 1 |eV^-1
%
KfnQP_Wc_dos= 0.000000     eV    # [EXTQP BSK BSS] W dos pre-factor  (conduction)
#NoCondSumRule                 # [BSE/X] Do not impose the conductivity sum rule in velocity gauge
#MetDamp                       # [BSE] Define \w+=sqrt(\w*(\w+i\eta))
BSEprop= "abs"                   # [BSS] abs/kerr/magn/dichr trace
% PL_weights
 1.000000 | 1.000000 | 1.000000 |        # [PL] [cc] Weights of the carthesian components of the emitted radiation
%
DrudeWBS= ( 0.000000 , 0.000000 )  eV    # [BSE] Drude plasmon
#Reflectivity                  # [BSS] Compute reflectivity at normal incidence
BoseCut= 0.100000                # [BOSE] Finite T Bose function cutoff
% BSEQptR
 1 | 1 |                             # [BSK] Transferred momenta range
%
% BSEBands
   49 | 56 |                         # [BSK] Bands range
%
BSKCut= 0.000000                 # [BSK] Cutoff on the BSE Kernel, 0=full 1=none
% BSEEhEny
-1.000000 |-1.000000 |         eV    # [BSK] Electron-hole energy range
%
% BSehWind
 100.0000 | 100.0000 |               # [BSK] [o/o] E/h coupling pairs energy window
%
% BEnRange
  -2.00000 | 5.00000 |         eV    # [BSS] Energy range
%
% BDmRange
 0.100000 | 0.100000 |         eV    # [BSS] Damping range
%
BDmERef= 0.000000          eV    # [BSS] Damping energy reference
BEnSteps= 1000                    # [BSS] Energy steps
% BLongDir
 1.000000 | 0.000000 | 0.000000 |        # [BSS] [cc] Electric Field
%
WRbsWF                        # [BSS] Write to disk excitonic the WFs
#BSSPertWidth                  # [BSS] Include QPs lifetime in a perturbative way
XfnQPdb= "none"                  # [EXTQP Xd] Database action
XfnQP_INTERP_NN= 1               # [EXTQP Xd] Interpolation neighbours (NN mode)
XfnQP_INTERP_shells= 20.00000    # [EXTQP Xd] Interpolation shells (BOLTZ mode)
XfnQP_DbGd_INTERP_mode= "NN"     # [EXTQP Xd] Interpolation DbGd mode
% XfnQP_E
 0.297000 | 1.000000 | 1.000000 |        # [EXTQP Xd] E parameters  (c/v) eV|adim|adim
%
XfnQP_Z= ( 1.000000 , 0.000000 )         # [EXTQP Xd] Z factor  (c/v)
XfnQP_Wv_E= 0.000000       eV    # [EXTQP Xd] W Energy reference  (valence)
% XfnQP_Wv
 0.000000 | 0.000000 | 0.000000 |        # [EXTQP Xd] W parameters  (valence) eV| 1|eV^-1
%
XfnQP_Wv_dos= 0.000000     eV    # [EXTQP Xd] W dos pre-factor  (valence)
XfnQP_Wc_E= 0.000000       eV    # [EXTQP Xd] W Energy reference  (conduction)
% XfnQP_Wc
 0.000000 | 0.000000 | 0.000000 |        # [EXTQP Xd] W parameters  (conduction) eV| 1 |eV^-1
%
XfnQP_Wc_dos= 0.000000     eV    # [EXTQP Xd] W dos pre-factor  (conduction)
% QpntsRXs
   1 | 274 |                         # [Xs] Transferred momenta
%
% BndsRnXs
   1 | 200 |                         # [Xs] Polarization function bands
%
NGsBlkXs= 7                Ry    # [Xs] Response block size
GrFnTpXs= "T"                    # [Xs] Green`s function (T)ordered,(R)etarded,(r)senant,(a)ntiresonant [T, R, r, Ta, Ra]
% DmRngeXs
 0.100000E-2 | 0.100000E-2 |   eV    # [Xs] Damping range
%
CGrdSpXs= 100.0000               # [Xs] [o/o] Coarse grid controller
% EhEngyXs
-1.000000 |-1.000000 |         eV    # [Xs] Electron-hole energy range
%
% LongDrXs
 1.000000 | 0.000000 | 0.000000 |        # [Xs] [cc] Electric Field
%
DrudeWXs= ( 0.000000 , 0.000000 )  eV    # [Xs] Drude plasmon
XTermKind= "none"                # [X] X terminator ("none","BG" Bruneval-Gonze)
XTermEn= 40.00000          eV    # [X] X terminator energy (only for kind="BG")

[Daniele Varsano]

Dear Shan Dong,

can you please attach the file instead of copy/paste, you need to rename them (e.g. .txt). It could be useful to look also at the report file.

At first sight, I cannot see anything wrong, just the Lz box side=29.000000 it could be quite small, but I do not know which is the size of your cell.
Anyway, I would check if the scissor 0.3 make sense, e.g. a GW calculation for the gap can be useful.
Also, I would check the convergence wrt BSEBands

Best,
Daniele

[shan dong]

Dear Shan Dong,

can you please attach the file instead of copy/paste, you need to rename them (e.g. .txt). It could be useful to look also at the report file.

At first sight, I cannot see anything wrong, just the Lz box side=29.000000 it could be quite small, but I do not know which is the size of your cell.
Anyway, I would check if the scissor 0.3 make sense, e.g. a GW calculation for the gap can be useful.
Also, I would check the convergence wrt BSEBands

Best,
Daniele

Dear Daniele,
Here are my input and output files. The scissor operator is the corrected value of my GW bandgap.

[Daniele Varsano]

Dear Shan Dong,

is the system you are dealing with T'-MoS2? In this case, you can experience an excitonic instability, please have a look at:
https://www.nature.com/articles/s41565-020-0650-4

If this is the case, please note:
1) A very dense k grid is needed to get converged calculations (both GW and BSE)
2) Scissor operator is not a good approximation for this system, GW correction for the direct and indirect gaps are very different.

Best,
Daniele