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Dear Juliana, <br>
<br>
In principle, with the flag "assume_isolated= 2D", there should be
nothing to take out from the band energies. That is, the vacuum
level is zero. The flag is designed to give the result one would get
with the 3D code in the limit of infinite distance between the
periodic images. <br>
<br>
A way to get the same number with the 3D code is, as you do, to
subtract the vacuum level. The fact that 2D band energies are
equivalent to the 3D ones minus the vacuum level has been tested on
a quite large number of 2D materials by some people in my group. It
works well, except when there are out-of-plane polarization or other
peculiarities in the electrostatics, which should not be your case.<br>
<br>
In your case, given the 90 Ang you use, you should be relatively
close to the limit of infinite distance between periodic image
limit, at least within energies much smaller than 1eV! In fact, I
would expect that you get the same band energies within some meV
with or without the flag...<br>
<br>
So... There is definitely a problem here. Could you send input,
output, version of the code, etc... so I could have a closer look?<br>
<br>
Thanks, <br>
Thibault Sohier<br>
THEOS, EPFL, Lausanne<br>
<br>
<br>
<div class="moz-cite-prefix">On 19/02/2019 12:00,
<a class="moz-txt-link-abbreviated" href="mailto:users-request@lists.quantum-espresso.org">users-request@lists.quantum-espresso.org</a> wrote:<br>
</div>
<blockquote type="cite"
cite="mid:mailman.1.1550574001.17424.users@lists.quantum-espresso.org">
<pre wrap="">Date: Mon, 18 Feb 2019 14:58:10 +0100
From: Juliana Morbec <a class="moz-txt-link-rfc2396E" href="mailto:jmmorbec@gmail.com" moz-do-not-send="true"><jmmorbec@gmail.com></a>
To: <a class="moz-txt-link-abbreviated" href="mailto:users@lists.quantum-espresso.org" moz-do-not-send="true">users@lists.quantum-espresso.org</a>
Subject: [QE-users] about the option "assume_isolated=2D"
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Dear all.
I have been trying to compute the band edge of 2D TMDCs and I have tested
the option with and without "assume_isolated=2D". I noticed that this flag
strongly affects the positions of the band edges. For example, the VBM/CBM
of MoS2 change from -5.93/-4.20 eV with "assume_isolated=2D" to -4.96/-3.23
eV without "assume_isolated=2D"; all values were computed with respect to
the vacuum level, which is 0 eV in the first case and 0.08 eV in the second
case; I used a quite significant vacuum area in the calculatons (~90 Ang).
I read the paper "Density functional perturbation theory for gated
two-dimensional heterostructures: Theoretical developments and application
to flexural phonons in graphene" by Sohier, Calandra add Mauri (Physical
Review B 96, 75448), and I understood that the new method would make
differences if one wants to investigate charged 2D materials, e.g. with
charged defect, or 2D materials in a perpendicular electric field, but not
in the case of pure MoS2, no charge, no field. I do not understand why such
a large difference in the case of pure 2D layers.
I will appreciate if someone could comment on this.
Thank you for your time.
Best regards,
Juliana Morbec
<div class="moz-txt-sig">--
Juliana Morbec, PhD
Research Associate - Prof. Kratzer's group
University of Duisburg-Essen, Germany
<a class="moz-txt-link-freetext" href="https://jmmorbec.wordpress.com/" moz-do-not-send="true">https://jmmorbec.wordpress.com/</a></div></pre>
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