<div dir="ltr"><p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">Dear all<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><b>Problem in brief:</b> 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.<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><b>Questions:<span></span></b></p>
<ol type="1" style="margin-top:0in;margin-bottom:0in" start="1">
<li class="gmail-MsoListParagraph" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span>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?<span></span></span></li>
<li class="gmail-MsoListParagraph" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span>Is there any problem
with my input?<span></span></span></li>
</ol>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><b>Details of the simulation:</b> 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. <span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="ES">&control<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="ES">
calculation='scf',<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> outdir = '/home/tmp/MoS2.2L.E2.01/'<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> <span lang="ES">prefix='MoS2.2L',<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="ES"> pseudo_dir
= '/home/pseudo/',<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="ES"> </span>restart_mode='from_scratch',<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> tprnfor=.true.<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> tefield=.true.<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">/<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">&system<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> ibrav=4,<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> celldm(1)= 6.02820958<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> celldm(3)= 6.629172038<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> nat=6,<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> ntyp= 2,<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> ecutwfc= 80.0,<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> nbnd=24<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> vdw_corr='DFT-D'<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> edir=3<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> eamp = 0.00388938<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> <span lang="FR">emaxpos=0.60<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="FR">
eopreg=0.10<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="FR">/<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="FR">&electrons<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="FR"> conv_thr=
1.0d-9,<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="FR"> </span>mixing_beta=
0.6,<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">/<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">ATOMIC_SPECIES<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> Mo 95.94 Mo.pbe-mt_fhi.UPF<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> S 32.065 S.pbe-mt_fhi.UPF<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">ATOMIC_POSITIONS {alat}<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> Mo
0.000000000 0.000000000 0.000000000 <span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> S
0.500000000 0.288675135 -0.492491069<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> S
0.500000000 0.288675135 0.492515463<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> Mo
0.500000000 0.288675135 1.969804069<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> S
-0.000000000 -0.000000000 1.477442788<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> S
-0.000000000 -0.000000000
2.462449934<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">K_POINTS {automatic}<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> 13 13 1 0 0 0<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"> <span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><b>Details of the problem:</b> 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.<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><b>References:<span></span></b></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">[1] Ramasubramaniam A. et al, “Tunable band gaps in bilayer
transition-metal dichalcogenides”, PHYSICAL REVIEW B 84, 205325 (2011)<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">[2] Chu T. et al, “Electrically Tunable Bandgaps in Bilayer
MoS2”, Nano Lett. 2015, 15, 8000−8007<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">[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<span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt">[4] Santos E., et al, “Electrically Driven Tuning of the
Dielectric Constant in MoS2 Layers”, ACS NANO, VOL. 7, NO. 12,
10741–10746 ’ 2013<b><span></span></b></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span> </span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="ES">Carlos Polanco<span></span></span></p>
<p class="MsoNormal" style="margin:0in;font-family:"Calibri",sans-serif;font-size:11pt"><span lang="ES">Scientist at
Sivananthan Laboratories<span></span></span></p>
<br clear="all"><br>-- <br><div class="gmail_signature" dir="ltr" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div>Carlos Andrés Polanco García</div></div></div></div></div></div></div>