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Hey Cameron,<br>
<br>
I think that the main problem is your lattice parameter.<br>
You use 5.8364 bohr which is 5.8364*0.529177 = 3.088 Angstrom.<br>
Thus, your MoS2 is under compressive strain. That's why you<br>
(correctly) get an indirect band gap between K and Q.<br>
See also Fig. 3(b) of Phys. Rev. B 85, 033305:<br>
<br>
<a class="moz-txt-link-freetext" href="http://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.033305">http://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.033305</a><br>
<br>
Cheerio<br>
<br>
Thomas<br>
<br>
<div class="moz-cite-prefix">On 09/24/2015 08:20 AM, Cameron Foss
wrote:<br>
</div>
<blockquote
cite="mid:CANUcA0M-6GzLXMS8MBQSXdJq6AUGfhQHhkpmmO6x+jikQWdVXg@mail.gmail.com"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=windows-1252">
<div dir="ltr">Hello,
<div><br>
</div>
<div>I am attempting to obtain the band structure of monolayer
MoS2, however my results so far do not agree well with what
has been documented for monolayer (ML) MoS2. I consistently
get an indirect band gap for ML MoS2 caused by the 'Q-valley'
minimum seen in bulk MoS2.</div>
<div><br>
</div>
<div>
<div>I have tested the code with interplanar distances of 15
and 30 angstroms to eliminate any interplanar interactions.
There have been several papers that report the band
structure for ML MoS2 from first principles, they have used
functionals from LDA, GGA, and an HSE hybrid functional. My
best success has been with GGA-PW91(see input file below).
Note in all my simulations I have measured a band gap of
~1.9eV which is in agreement with reported values.</div>
<div><br>
</div>
<div>I am uncertain as to what the issue could be? I have
tested 3 different PPs. </div>
</div>
<div><br>
</div>
<div>%%%%%%%%%%%%%%%%%%%%</div>
<div>Details on simulations:</div>
<div><br>
</div>
<div> Specifically, my simulations continue to show that the
'Q-valley' is slightly lower (16meV~30meV) than the minimum at
the K point, suggesting an indirect band gap. However ML mos2
has been widely shown to have a direct band gap at the K point
where this 'Q-valley' along the conduction band is suppressed
above the K-point minimum.</div>
<div><br>
</div>
<div>I run a scf (pw.x) calculation first, followed by an nscf
(pw.x) run, and a bands calculation with pw.x (and of course
bands.x to retrieve the energy values). I specify 16 bands for
nbnd, kpoints along symmetry paths, and a MP grid of 8x8x1
with a 1 1 1 offset. </div>
<div><br>
</div>
<div>Input file: </div>
<div><br>
</div>
<div>
<div>&control</div>
<div> calculation='scf' !nscf, then
bands</div>
<div> restart_mode='from_scratch',</div>
<div> !pseudo_dir='directory where pseudopotentials are
stored/',</div>
<div> !outdir='directory where large files are written/'</div>
<div> pseudo_dir='/home/cameron/QE/espresso-5.1/pseudo/',</div>
<div> outdir='/home/cameron/QE/espresso-5.1/2dout'</div>
<div> prefix='mos2-gga',</div>
<div> /</div>
<div> &system</div>
<div> ibrav=4, celldm(1)=5.8364, celldm(3)=10,</div>
<div> nat=3, ntyp=2, ecutwfc =70, ecutrho=300 !
specified nbnd=16</div>
<div> /</div>
<div> &electrons</div>
<div> conv_thr = 1.0d-15 ! reduced to
1.0d--12 for nscf, bands</div>
<div> mixing_beta = 0.5 ! increased to
0.7 for nscf, bands</div>
<div> /</div>
<div>ATOMIC_SPECIES</div>
<div> Mo 95.94 Mo.pw91-n-van.UPF</div>
<div> S 32.065 S.pw91-n-mt.UPF</div>
<div>ATOMIC_POSITIONS bohr</div>
<div>S 0.00002 -0.00001 0.000000000</div>
<div>Mo 2.91822 1.68481 2.929886569</div>
<div>S 0.00002 -0.00001 5.859870374</div>
<div>K_POINTS automatic</div>
<div> 8 8 1 1 1 1</div>
</div>
<div><br>
</div>
<div>I ran into some convergence issues with all the bands with
this conv_thr in the nscf and bands calculations, so i had to
reduce it to 1.0d-12 for those calculations. I also used a
mixing beta of 0.7 for both the nscf and bands calculations. I
am uncertain as to whether changing mixing_beta's between
calculations is of importance or not as well, but I do not
think this is the case.</div>
<div><br>
</div>
<div>Best regards,</div>
<div>Cameron</div>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
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</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Dr. rer. nat. Thomas Brumme
Max Planck Institute for the Structure and Dynamics of Matter
Luruper Chaussee 149
22761 Hamburg
Tel: +49 (0)40 8998 6557
email: <a class="moz-txt-link-abbreviated" href="mailto:Thomas.Brumme@mpsd.mpg.de">Thomas.Brumme@mpsd.mpg.de</a>
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