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 0.34200

2180 2.31460 2.57420 0.25960

[b]2594 2.31650 2.57100 0.25450[/b]

3044 2.31000 2.57630 0.26630

4052 2.29280 2.59580 0.30300

4901 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...

Thanks in advance for your help and insight!

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] Optics

chi # [R CHI] Dyson equation for Chi.

Chimod= "IP" # [X] IP/Hartree/ALDA/LRC/BSfxc

NGsBlkXd= 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

%