Dear Juliana,
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.
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.
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...
So... There is definitely a problem here. Could you send input, output,
version of the code, etc... so I could have a closer look?
Thanks,
Thibault Sohier
THEOS, EPFL, Lausanne
On 19/02/2019 12:00, users-requ...@lists.quantum-espresso.org wrote:
Date: Mon, 18 Feb 2019 14:58:10 +0100
From: Juliana Morbec<jmmor...@gmail.com>
To:users@lists.quantum-espresso.org
Subject: [QE-users] about the option "assume_isolated=2D"
Message-ID:
<caffgdao2s_frfxynk+mzjye_tdqf3zvenqitdpeccevtdye...@mail.gmail.com>
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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
-- Juliana Morbec, PhD Research Associate - Prof. Kratzer's group
University of Duisburg-Essen, Germany https://jmmorbec.wordpress.com/
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