Yambo Community Forum

I want to do a calculation based on GW+BSE. So, I did the following steps:

1-./yambo -g n -p p
2-./yambo -b
3. yambo -o b -k sex -y d

However, the final obsorption spectrum is same to this calculation without GW correction (namely, only step 2 and 3). HOw to enable the GW's result on the final absorption spectrum? Of couse, If this system is a semicoducting material, we can add KfnQP_E parameter in BSE calculation; while for a metalic system, how to achieve this.

Dear Ljzhou,
the QP corrections are inserted exactly in the same way using :
KfnQPdb= "E < ./SAVE/ndb.QP" or db.QP depending if you are using or not the netcdf libraries.
in this way you add the QP corrections calculated in the GW run for each bands and stored in the QP database.
For metal, you can also consider to include intraband transition using the Drude Model with the variable DrudeWBS.

Best,
Daniele

"For metal, you can also consider to include intraband transition using the Drude Model with the variable DrudeWBS."

How to use the variable DrudeWBS? By adding DrudeWBS =(0, 0) or other values? What's the mean of the two values in parentheses?

Dear Zhou,

How to use the variable DrudeWBS? By adding DrudeWBS =(0, 0) or other values? What's the mean of the two values in parentheses?

Please have a look in the textbook what the Drude model is. The two fields represent the real and imaginary part of the Drude frequency. The imaginary part corresponds to the width of the peak appearing in the EELS. Of course if you leave it to zero this has no effect. The values of the Drude frequency for your system has to be taken from the literature.
Best,
Daniele

Thank U for your interpretation. There is a serious question for the result of 2D silicene.

Namely, there is a big difference on the absorption onset between my reluts of GW+BSE and the reference ( Wei, W.; Dai, Y.; Huang, B.; Jacob, T. Phys. Chem. Chem. Phys. 2013, 15, 8789–8794.). What's more, the peak positions also have big difference. The comparison is showed the the figure attachments. For my calculation, the yambo.in is as followed:
optics # [R OPT] Optics
bse # [R BSE] Bethe Salpeter Equation.
bsk # [R BSK] Bethe Salpeter Equation kernel
bss # [R BSS] Bethe Salpeter Equation solver
rim_cut # [R RIM CUT] Coulomb interaction
em1s # [R Xs] Static Inverse Dielectric Matrix
KfnQPdb= "E < ./SAVE/ndb.QP"
CUTGeo= "box z" # [CUT] Coulomb Cutoff geometry: box/cylinder/sphere
% CUTBox
0.00000 | 0.00000 | 12.00000 | # [CUT] [au] Box sides
%
#CUTRadius= 0.000000 # [CUT] [au] Sphere/Cylinder radius
#CUTCylLen= 0.000000 # [CUT] [au] Cylinder length
Chimod= "Hartree" # [X] IP/Hartree/ALDA/LRC/BSfxc
BSEmod= "coupling" # [BSE] resonant/causal/coupling
BSKmod= "SEX" # [BSE] IP/Hartree/HF/ALDA/SEX
BSSmod= "d" # [BSS] (h)aydock/(d)iagonalization/(i)nversion/(t)ddft`
BSENGexx= 30249 RL # [BSK] Exchange components
BSENGBlk= 205 RL # [BSK] Screened interaction block size
DrudeWBS=(0, 0)
WehCpl # [BSK] eh interaction included also in coupling
% BEnRange
0.00000 | 10.00000 | eV # [BSS] Energy range
%
% BDmRange
0.10000 | 0.10000 | eV # [BSS] Damping range
%
RandQpts= 3000000 # [RIM] Number of random q-points in the BZ
RandGvec= 1 RL # [RIM] Coulomb interaction RS components

BEnSteps= 500 # [BSS] Energy steps
% BLongDir
1.000000 | 0.000000 | 0.000000 | # [BSS] [cc] Electric Field
%
% BSEBands
2 | 8 | # [BSK] Bands range
%
% QpntsRXs
1 | 43 | # [Xs] Transferred momenta
%
% BndsRnXs
1 | 20 | # [Xs] Polarization function bands
%
NGsBlkXs= 205 RL # [Xs] Response block size
% LongDrXs
1.000000 | 0.000000 | 0.000000 | # [Xs] [cc] Electric Field
%


What's the reason for such big difference? Are there any errs in my input file for yambo calculation?

The literature calculation details are as follows:" The static screening in the direct term is calculated within the random- phase approximation (RPA).37 Only the resonant part of the Bethe–Salpeter Hamiltonian is considered, i.e., Tamm–Dancoff approximation,38 with which the non-Hermitian BSE is reduced to a Hermitian one and can be solved by an efficient iterative method. In both GW and BSE calculations, screened Coulomb interactions were truncated due to their long-range nature; otherwise, unphysical interactions between periodic images would result.39 The Kohn–Sham DFT results are obtained by using the ABINIT40 code, while GW + BSE calculations are performed using the YAMBO program suite.41"

Dear Zhou Liu,
I can see discrepancies from the paper you mentioned at RPA level, so it has not to do with coupling/resonant approach.
My suggestion is:

1. Look if you ground state band structure is in agreement with the paper you mentioned (it is published),
2. Check all your convergence parameter (k-points,Gvecs etc.),
3. Write to the authors to ask details (e.g; atomic positions, Drude contributions if any, etc.).

I do not know your structure, if you have simmetries etc, consider that in the absorption spectra also the y-direction should be considered, unless you have symmetries such that the x and y direction a re the same.

Please also note that when using the box cutoff, the interaction length of the coulomb cutoff is Lc/2, so in your case 6a.u., this is something that has to do with the yambo subroutine. Check if this what you want (it dependes on your supercell size).

Again, my suggestion is to concentrate first with the ground state band structure and RPA spectrum before passing to the BSE.

Regards,
Daniele