## IP spectra of bilayer graphene - choice of k-points

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, andrea marini, Conor Hogan, myrta gruning

### IP spectra of bilayer graphene - choice of k-points

Dear all,

I am working on bilayer graphene with finite electric field; this system has a "real" gap (~0.3-0.4 eV) and I want to see its low-energy optical spectrum (basically to see how doping affects the spectrum); however I find it extremely hard to find a k-point grid that gives a "converged" spectrum.

Since applying a field splits the "Dirac cone" into two separate "cones" in k-space I was first looking into finding the k-grid that gives the smallest band-gap (i.e. this really hits the VBM and CBM). All my grids are divisible by 3 (i.e. they also contain the K point which has a significant transition amplitude). For grids 9,33,66,72,78.. the band-gap (last column) is

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`  41 2.06940 2.79530 0.72590 545 2.27250 2.61450 0.342002180 2.31460 2.57420 0.25960[b]2594 2.31650 2.57100 0.25450[/b]3044 2.31000 2.57630 0.266304052 2.29280 2.59580 0.303004901 2.30970 2.57900 0.26930`

Now the gap of grids with 66, 72 and 78 k-points look quite similar but their spectra look quite different; Part of it might just be due to smearing but I was wondering if there is a better way of testing this? Since I will change the field and the doping level testing this every time will be very cumbersome...

Chris

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`# GPL Version 4.2.1 Revision 110. (Based on r.14778 h.7b4dc3)#                        MPI Build                          #                http://www.yambo-code.org                  #optics                         # [R OPT] Opticschi                            # [R CHI] Dyson equation for Chi.Chimod= "IP"                   # [X] IP/Hartree/ALDA/LRC/BSfxcNGsBlkXd= 1            Ry      # [Xd] Response block size% QpntsRXd    1 |  1 |                 # [Xd] Transferred momenta%% BndsRnXd   1 |  120 |                   # [Xd] Polarization function bands%% EnRngeXd  0.00000 | 5.00000 | eV      # [Xd] Energy range%% DmRngeXd  \$smear |  \$smear | eV      # [Xd] Damping range%ETStpsXd= 1000                  # [Xd] Total Energy steps% LongDrXd 0.000000 | 0.000000 | 1.000000 |        # [Xd] [cc] Electric Field%`
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Christoph Wolf

Postech university, South Korea
chwolf@postech.ac.kr
chwolf

Posts: 12
Joined: Mon Jul 11, 2016 2:11 pm
Location: South Korea

### Re: IP spectra of bilayer graphene - choice of k-points

Dear Christoph,
actually, I do not have a clear suggestion for that. It really seems that you need a large k point sampling, besides that my suggestion is to increase the sampling by doubling it (when possible, ie having the smaller sampling as a subset of the larger one) and look at the spectrum considering larger smearing.

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/

Daniele Varsano

Posts: 1950
Joined: Tue Mar 17, 2009 2:23 pm