Hey Cameron,
I think that the main problem is your lattice parameter.
You use 5.8364 bohr which is 5.8364*0.529177 = 3.088 Angstrom.
Thus, your MoS2 is under compressive strain. That's why you
(correctly) get an indirect band gap between K and Q.
See also Fig. 3(b) of Phys. Rev. B 85, 033305:
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.033305
Cheerio
Thomas
On 09/24/2015 08:20 AM, Cameron Foss wrote:
Hello,
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.
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.
I am uncertain as to what the issue could be? I have tested 3
different PPs.
%%%%%%%%%%%%%%%%%%%%
Details on simulations:
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.
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.
Input file:
&control
calculation='scf' !nscf, then bands
restart_mode='from_scratch',
!pseudo_dir='directory where pseudopotentials are stored/',
!outdir='directory where large files are written/'
pseudo_dir='/home/cameron/QE/espresso-5.1/pseudo/',
outdir='/home/cameron/QE/espresso-5.1/2dout'
prefix='mos2-gga',
/
&system
ibrav=4, celldm(1)=5.8364, celldm(3)=10,
nat=3, ntyp=2, ecutwfc =70, ecutrho=300 ! specified nbnd=16
/
&electrons
conv_thr = 1.0d-15 ! reduced to 1.0d--12 for
nscf, bands
mixing_beta = 0.5 ! increased to 0.7 for nscf,
bands
/
ATOMIC_SPECIES
Mo 95.94 Mo.pw91-n-van.UPF
S 32.065 S.pw91-n-mt.UPF
ATOMIC_POSITIONS bohr
S 0.00002 -0.00001 0.000000000
Mo 2.91822 1.68481 2.929886569
S 0.00002 -0.00001 5.859870374
K_POINTS automatic
8 8 1 1 1 1
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.
Best regards,
Cameron
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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: [email protected]
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