[QE-users] Discrepancy between QE and VASP when including Efield in slab system

[Michal Krompiec]

Dear Carlos,
First of all, version 5.4 is ancient. Have you tried with the latest QE?
Assuming both calculations are performed correctly, the difference may stem
from different pseudopotentials. Are you using some old norm-conserving
pseudos? Are you sure your cutoff is large enough? Have you tried SSSP
pseudos?
Anyway, are you expecting reasonable results for this system from PBE?
Best regards,
Michal Krompiec, Merck Electronics

On Mon, Mar 8, 2021 at 6:40 PM Carlos Polanco Garcia <cap3fe at virginia.edu>
wrote:

> Dear all
>
>
>
> *Problem in brief:* My calculations of the change in the bandgap with
> electric filed in bilayer MoS2 using Quantum Espresso (QE) version 5.4.0 do
> not agree with similar calculations in the literature that used VASP.
>
>
>
> *Questions:*
>
>    1. Have anyone found discrepancies of the change of dispersion with
>    electric filed from QE and VASP in slab systems including Electric fields
>    with the sawtooth potential?
>    2. Is there any problem with my input?
>
>
>
> *Details of the simulation:* An input for bilayer MoS2 with an Electric
> field of 2V/nm is below. I set the discontinuity of the sawtooth potential
> to be in vacuum and I set the magnitude of the electric filed using the
> conversion factor 1 a.u. = 51.4220632*10^10 V/m.
>
>
>
> &control
>
>   calculation='scf',
>
>   outdir = '/home/tmp/MoS2.2L.E2.01/'
>
>   prefix='MoS2.2L',
>
>  pseudo_dir = '/home/pseudo/',
>
>   restart_mode='from_scratch',
>
>   tprnfor=.true.
>
>   tefield=.true.
>
> /
>
> &system
>
>   ibrav=4,
>
>   celldm(1)=    6.02820958
>
>   celldm(3)=   6.629172038
>
>   nat=6,
>
>   ntyp= 2,
>
>   ecutwfc=  80.0,
>
>   nbnd=24
>
>   vdw_corr='DFT-D'
>
>   edir=3
>
>   eamp =   0.00388938
>
>   emaxpos=0.60
>
>   eopreg=0.10
>
> /
>
> &electrons
>
>   conv_thr= 1.0d-9,
>
>   mixing_beta= 0.6,
>
> /
>
>
>
> ATOMIC_SPECIES
>
>   Mo  95.94    Mo.pbe-mt_fhi.UPF
>
>   S   32.065   S.pbe-mt_fhi.UPF
>
>
>
> ATOMIC_POSITIONS {alat}
>
>   Mo       0.000000000   0.000000000   0.000000000
>
>   S        0.500000000   0.288675135  -0.492491069
>
>   S        0.500000000   0.288675135   0.492515463
>
>   Mo       0.500000000   0.288675135   1.969804069
>
>   S       -0.000000000  -0.000000000   1.477442788
>
>   S       -0.000000000  -0.000000000   2.462449934
>
>
>
> K_POINTS {automatic}
>
>   13 13 1 0 0 0
>
>
>
> *Details of the problem:* Overall the dispersion for Efield=0 agrees
> decently with other calculations in the literature including those using
> VASP. However when I increase the electric field, my calculations predict a
> change in the Bandgap with electric field about four times weaker than that
> predicted from VASP calculations. For instance, from E0=0V/nm to E2=2V/nm,
> my bandgap (Gamma to K) decreases by 0.19 eV. On the other hand, similar
> VASP calculations predict that the bandgap decreases by 0.89 eV (Figure 2,
> red dispersions from [1]), and by 0.73 eV (Figure 5a from [2]). My
> calculations agree better with calculations that do not relay on VASP. An
> independent calculation with QE predicts a similar slope in the change of
> Bandgap with Efield for Efileds larger than 1V/nm (red dots Fig 3a [3]). A
> calculation with Siesta predicts a decrease in bandgap of 1.04 eV when
> increasing Efiled from 0V/nm to 5V/nm (Fig 4 b,d orange lines [4]), while
> my calculations predict a change in bandgap of 0.78 eV (20% difference).
> The agreement of the dispersions at Efield=0V/nm points that the problem is
> in how the Efiled is being included. My input for QE may be missing
> something, or there may be a factor difference in how Efield is included
> between VASP and QE.
>
>
>
> *References:*
>
> [1] Ramasubramaniam A. et al, “Tunable band gaps in bilayer
> transition-metal dichalcogenides”, PHYSICAL REVIEW B 84, 205325 (2011)
>
> [2] Chu T. et al, “Electrically Tunable Bandgaps in Bilayer MoS2”, Nano
> Lett. 2015, 15, 8000−8007
>
> [3] Nguyen C. et al, “Band Gap Modulation of Bilayer MoS2 Under Strain
> Engineering and Electric Field: A Density Functional Theory”, Journal of
> ELECTRONIC MATERIALS, Vol. 45, No. 8, 2016
>
> [4] Santos E., et al, “Electrically Driven Tuning of the Dielectric
> Constant in MoS2 Layers”, ACS NANO, VOL. 7,  NO. 12,  10741–10746 ’ 2013
>
>
>
> Carlos Polanco
>
> Scientist at Sivananthan Laboratories
>
>
>
> --
> Carlos Andrés Polanco García
> _______________________________________________
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