strange gw+bse results on two-layered 2D materials

Concerns issues with computing quasiparticle corrections to the DFT eigenvalues - i.e., the self-energy within the GW approximation (-g n), or considering the Hartree-Fock exchange only (-x)

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strange gw+bse results on two-layered 2D materials

Postby ljzhou86 » Sun Dec 10, 2017 7:50 am

Dear Developers,

Recently, I am using yambo to perform gw+bse calculations on a 2D material, considering the layer-depended excitonic effect. As shown in the follwoing figures, I summarized the main results as follows,

single-layer (1L): DFT gap:0.8508 eV; G0W0 gap: 2.06 eV; the first gw+BSE optical exciton peak: 1.48 eV, so the exciton binding energy: 2.06—1.48=0.58 eV
double-layer (2L): DFT gap: 0.8289 eV; G0W0 gap: 1.9 eV; the first gw+BSE optical exciton peak: 1.96 eV, so the exciton binding energy: 1.9 — 1.96 = —0.06 eV
Triple-layer (3L): DFT gap: 0.8142 eV; G0W0 gap: 1.65 eV; the first gw+BSE optical exciton peak: 1.44 eV, so the exciton binding energy: 1.65 — 1.44 = 0.21 eV


It seems the gw+bse results for 2L are very strange: blue-shifted first optical exciton peak and thus negative exciton binding energy. I have double checked the results and parameters used in my calculations, but I am still confused why I got so strange results for 2L. I attached the pwscf and yambo inputs and r_* log files for 2L calculations. Could you help me to pinpoint this problem?

Thanks a lot,
Liujiang Zhou
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Last edited by ljzhou86 on Sun Dec 10, 2017 7:54 am, edited 1 time in total.
Dr. Zhou Liu-Jiang
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Fuzhou, Fujian, 350002
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Re: strange gw+bse results on two-layered 2D materials

Postby ljzhou86 » Sun Dec 10, 2017 7:52 am

Sorry not to attach the input and oupout files, please see it in attachment.
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Dr. Zhou Liu-Jiang
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Fuzhou, Fujian, 350002
ljzhou86
 
Posts: 72
Joined: Fri May 03, 2013 10:20 am

Re: strange gw+bse results on two-layered 2D materials

Postby Daniele Varsano » Mon Dec 11, 2017 9:39 am

Dear Zhou,

from the output you posted I can't see the negative binding energy you mention: the red line (BSE) is always al lower energy than the green line (IP).
In order to check the exact excitation energy you should diagonalise the excitonic matrix in order to have access to the eigenvalues: you have a 20K dimension matrix, even if a bit hard should be still possible to diagonalise (use "d" instead of "h") in the BSSmod variable.
If the trend is not what you expect it is recommendable to check the convergence parameters.
Please note that in the calculation of the screening you did not use the coulomb cutoff potential:
Code: Select all
   [RD./SAVE//ndb.em1s]----------------------------------------
    Brillouin Zone Q/K grids (IBZ/BZ):   61   576    61   576
    RL vectors                   (WF): 10503
   *WRN* Coulomb cutoff potential         :none


and I think it should activated for consistency with the rest of the calculation.

Best,

Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/
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