[Wannier] band gap of bilayer graphene

[Roozbeh Anvari]

Dear Wanier users,

I figured the small band gap of about 40 meV was caused by a displacement
of about  0.001 Ang in the initial atomic coordinates of the second layer
from the center of the hexagon of the first layer, and can be easily fixed
by starting the ionic relaxation from a strictly symmetric structure !

Best,
Roozbeh


On Thu, Dec 31, 2020 at 7:29 PM Roozbeh Anvari <roozbeh.anvari at gmail.com>
wrote:

>
> Dear Wannier90 experts,
>
> I just posted this question on quantum-espresso's archive but it might be
> more helpful to ask it again here,
>
> I am trying to calculate the maximally localised wannier functions of
> defective bilayer graphene using quantumEspresso and Wannier90 codes,
>
> however I am getting a band gap of about 40 to 60 meV for pristine bilayer
> graphene using  a variety of NCPP+LDA and NCPP+PBE, the band gap and the
> interlayer distance are slightly improved (10 meV and 0.1 A ) after
> applying DFT-D3 but not significantly, I wonder how I can fix this issue
> (please see the sample setup that follows) ?
>
> Please also note that I get an ideal zero band gap when using ultrasoft
> pseudopotential but it seems that Wannier90 has a preference for NCPP and I
> cannot get a nice fit to the band structure calculated using USPP, while
> the calculated MLWF for  NCPP+PBE are amazingly perfect (total Omega = 6
> A^2)
>
> Best regards,
> Roozbeh Anvari,
> PDF, Physics Dept, Queen's University, Canada
>
> Here is a shortened sample of the setup I use for  vc-relax/scf
> calculations
>
> &CONTROL
> ...
>  forc_conv_thr=1.0d-6,
>
> &SYSTEM
> ...
> occupations='smearing', smearing='cold', degauss=0.02
> vdw_corr='grimme-d3' , input_dft='PBE'
>
> &ELECTRONS
> startingwfc='random', diagonalization='cg', conv_thr = 1.0e-12
> mixing_beta = 0.2
> /
>  &IONS
>  ion_dynamics='bfgs',  upscale=20.0
>
> &CELL
>  press_conv_thr = 0.5D0,  press = 0.D0
> cell_dynamics = 'bfgs',cell_dofree = '2Dxy',  cell_factor = 1.5D0
>
> ATOMIC_SPECIES
> C  12.011   C.pbe-mt_gipaw_NC_PBE.UPF
>
> K_POINTS automatic
> 36 36 1 0 0 0
>
> Cheers
>
>
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