[Wannier] band gap of bilayer graphene

[Roozbeh Anvari]

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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