Energy gap from optical absorption

Deals with issues related to computation of optical spectra, in RPA (-o c) or by solving the Bethe-Salpeter equation (-o b). Includes local field effects, excitons, etc.

Moderators: Davide Sangalli, Daniele Varsano, andrea.ferretti, andrea marini, Conor Hogan, myrta gruning

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DavidPolito93
Posts: 15
Joined: Sat Jun 06, 2020 10:43 am

Energy gap from optical absorption

Post by DavidPolito93 » Thu Jul 30, 2020 7:52 am

Dear developers,

I am treating a 1D system oriented along the z direction with the ws cutoff. I have computed the band structure within the G0W0 approximation and I obtain an energy gap of ~ 2 eV.
Then I computed the optical spectra (without BSE): Yambo gives me the alpha file (by the way, why does it have four columns? I would have expected only two, one for the imaginary part and one for the real part. Instead I have two imaginary parts and two real parts).

My problem is that I would have expected a peak around 2 eV for the imaginary part, but I get a peak at exactly half of the value of the energy gap (i.e. ~ 1 eV).
Is there some factor of 2 that have to be taken into account? Or am I doing something wrong?

I carefully checked the convergence for all of the parameters. Maybe I missed something?

I am attaching the input:

optics # [R OPT] Optics
rim_cut # [R RIM CUT] Coulomb potential
chi # [R CHI] Dyson equation for Chi.
dipoles # [R ] Compute the dipoles
DIP_Threads=0 # [OPENMP/X] Number of threads for dipoles
X_Threads=0 # [OPENMP/X] Number of threads for response functions
RandQpts=0 # [RIM] Number of random q-points in the BZ
RandGvec= 1 RL # [RIM] Coulomb interaction RS components
CUTGeo= "ws z" # [CUT] Coulomb Cutoff geometry: box/cylinder/sphere/ws X/Y/Z/XY..
CUTwsGvec= 1.100000 # [CUT] WS cutoff: number of G to be modified
Chimod= "HARTREE" # [X] IP/Hartree/ALDA/LRC/PF/BSfxc
NGsBlkXd= 3 Ry # [Xd] Response block size
% QpntsRXd
1 | 1 | # [Xd] Transferred momenta
%
% BndsRnXd
1 | 30 | # [Xd] Polarization function bands
%
% EnRngeXd
0.00000 | 40.00000 | eV # [Xd] Energy range
%
% DmRngeXd
0.20000 | 0.20000 | eV # [Xd] Damping range
%
ETStpsXd= 4001 # [Xd] Total Energy steps
% LongDrXd
0.000000 | 0.000000 | 1.000000 | # [Xd] [cc] Electric Field
%

Sincerely,

Davide Romanin
-----------------------------------------------------
PhD student in Physics XXXIII cycle
Representative of the PhD students in Physics
Applied Science and Technology department (DiSAT)
Politecnico di Torino
Corso Duca degli Abruzzi, 24
10129 Torino ITALY
------------------------------------------------------

Palummo Maurizia
Posts: 10
Joined: Wed Apr 01, 2009 8:10 am
Location: Dipartimento di Fisica Universita' Tor Vergata Roma
Contact:

Re: Energy gap from optical absorption

Post by Palummo Maurizia » Thu Jul 30, 2020 7:58 am

Dear Davide

you are obtaining 4 columns because you are doing an RPA + local-field calculation. the column 4 and 5 are the RPA without LF (real IP calculation) while 2 and 3
are with LF included.
Regarding the peak at 1 eV are you sure you are reading the GW corrections?

Best
Maurizia
Dipartimento di Fisica
Universita' 'Tor Vergata'
Via della Ricerca Scientifica I
tel.++39-06-72594894
fax.++39-06-2023507
http://www.fisica.uniroma2.it/~cmtheo-group/

ETSF European Theoretical Spectroscopy Facility www.etsf.eu

DavidPolito93
Posts: 15
Joined: Sat Jun 06, 2020 10:43 am

Re: Energy gap from optical absorption

Post by DavidPolito93 » Thu Jul 30, 2020 8:10 am

Dear Maurizia,

Thank you for your rapid reply!

Effectively, I checked and it is the PBE gap (the one without the GW corrections). I thought I gave the correct SAVE folder. I will try to redo the computation and I'll let you know

Thanks a lot,

Davide
-----------------------------------------------------
PhD student in Physics XXXIII cycle
Representative of the PhD students in Physics
Applied Science and Technology department (DiSAT)
Politecnico di Torino
Corso Duca degli Abruzzi, 24
10129 Torino ITALY
------------------------------------------------------

Palummo Maurizia
Posts: 10
Joined: Wed Apr 01, 2009 8:10 am
Location: Dipartimento di Fisica Universita' Tor Vergata Roma
Contact:

Re: Energy gap from optical absorption

Post by Palummo Maurizia » Thu Jul 30, 2020 8:37 am

Dear Davide

in any case you have to read the ndb.QP file or add the correction as scissor + streaching in the input file
if you want to do a calculation of epsilon2 at RPA+LF starting from the corrected bands.

Maurizia
Dipartimento di Fisica
Universita' 'Tor Vergata'
Via della Ricerca Scientifica I
tel.++39-06-72594894
fax.++39-06-2023507
http://www.fisica.uniroma2.it/~cmtheo-group/

ETSF European Theoretical Spectroscopy Facility www.etsf.eu

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Daniele Varsano
Posts: 2604
Joined: Tue Mar 17, 2009 2:23 pm
Contact:

Re: Energy gap from optical absorption

Post by Daniele Varsano » Thu Jul 30, 2020 11:01 am

Dear Davide,
in order to include your GW quasiparticle you need to add the following keywords in the input file:

Code: Select all

XfnQPdb= "E < ./dir/ndb.QP"
where dir is the directory containing the QP database (SAVE if you have not redirected in other directory).

Alternatively you can assign a scissor/streching correction by using:

Code: Select all

% XfnQP_E
 delta-E | slople_conduction | slope_valence |        # [EXTQP Xd] E parameters  (c/v) eV|adim|adim
%


These variables are activated adding the qp verbosity un the input file generation:
yambo -o c -V qp
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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