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<p>Hi,</p>
<p>besides the reply i) of Giovanni to your question 1), I would
like to add something about doped systems. The doping
concentration is defined by how many dopant atoms you have in your
unit cell; for this you might need to use supercells of the
primitive one, for sure if you substitute the Mo site. This means
that your doped system can be seen as a supercell of your
primitive undoped system; but this "supercell" is now a primitive
unit cell, as it fixes the dopant concentration and cannot be
reduced to a smaller unit cell. Since the matrix hosting the
dopant is obtained by replicating the undoped primitive cell, the
band structure (electronic or phononic) will show band folding
features that Giovanni mentioned. However, since now the unit cell
is primitive (again, because it is the minimum volume you need to
have the chosen dopant concentration), any possible change of the
band gap character from indirect to direct (and vice versa) is
real and not an artifact due to the band folding.</p>
<p>You may read the paper
<a class="moz-txt-link-freetext" href="https://doi.org/10.1016/j.diamond.2022.109237">https://doi.org/10.1016/j.diamond.2022.109237</a> for a discussion
about this.</p>
<p>I hope this helps.</p>
<p>Best</p>
<p>Antonio<br>
</p>
<div class="moz-cite-prefix">Il 30. 05. 25 9:51, Giovanni Cantele ha
scritto:<br>
</div>
<blockquote type="cite"
cite="mid:CAG=KO1HA0MJ8G4xy01_0U4aYWuU2DZp04ErKKhuLbG2ug+Yqfg@mail.gmail.com">
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<div>Hi,</div>
<div><br>
</div>
<div>first suggestion is start with "simple" calculations, then
move to trickier ones (such as phonon dispersions). </div>
<div>Second suggestion is to follow tutorials to be fully aware
of all the main issues you may face during a specific
calculation.</div>
<div><br>
</div>
<div>This being said, some considerations about your questions:</div>
<div><br>
</div>
<div>i) it is absolutely "normal" that if you replicate a unit
cell along one or more directions the number of bands
increases</div>
<div>proportionally to the number of unit cells, even if you are
making a calculation for a "perfect" undoped crystal.</div>
<div>You might want to study, for example, what happens to a one
dimensional monoatomic chain with period a when you study it </div>
<div>in a doubled unit cell with period 2a. This is something
you might find in textbooks and in web sites as well.</div>
<div>If your supercell is composed of N supercells of the
original lattice, for each band of the latter you will get N
bands of the former.</div>
<div>The explanation stands in the size of the Brillouin zone,
whose volume reduces by a factor N. However, the "perfect"
crystal with</div>
<div>its minimal primitive cell and the same with the supercell
will have exactly the same "physics". For example, you should
get</div>
<div>the same density of states, except for a factor N. What
I've explained is generally referred to as "band folding". The
change in the</div>
<div>volume and possibly the shape of the Brillouin zone also
explain the fact that some point of the original Brillouin
zone (e.g. K, M)</div>
<div>can be "folded" to a different point of the new Brillouin
zone. In the monoatomic chain above mentioned, for example,
the</div>
<div>Brillouin zone extremum pi/a gets folded to the gamma
point.</div>
<div><br>
</div>
<div>ii) "<span style="font-family:tahoma,sans-serif"> </span><span
style="font-family:tahoma,sans-serif">I am working with the
un-relaxed unitcell" : what does it mean? Did you optimize
the atomic positions? Calculation of phonon</span></div>
<div><span style="font-family:tahoma,sans-serif">frequencies is
based on the assumption that the starting configuration is
an equilibrium configuration, that is, zero forces.</span></div>
<div><span style="font-family:tahoma,sans-serif">So the first
step is to optimize the atomic positions, sometimes you need
to optimize them also with thresholds tighter than </span></div>
<div><span style="font-family:tahoma,sans-serif">those one
normally uses to get equilibrium geometries. Starting from a
non-equilibrium geometry means an unstable system and in
turn</span></div>
<div><span style="font-family:tahoma,sans-serif">this results in
negative phonon frequencies. Be careful to experience the
calculation of phonons on very simple systems, the
convergence</span></div>
<div><span style="font-family:tahoma,sans-serif">of the
frequencies is harder to obtain than other properties, such
as band structure or bond lengths.</span></div>
<div><span style="font-family:tahoma,sans-serif"><br>
</span></div>
<div><span style="font-family:tahoma,sans-serif">Giovanni</span></div>
<div><br>
</div>
<div>
<div dir="ltr" class="gmail_signature"
data-smartmail="gmail_signature">
<div dir="ltr"><span style="color:rgb(0,0,0)">-- <br>
<br>
Giovanni Cantele, PhD<br>
CNR-SPIN<br>
c/o Dipartimento di Fisica<br>
Universita' di Napoli "Federico II"<br>
Complesso Universitario M. S. Angelo - Ed. 6<br>
Via Cintia, I-80126, Napoli, Italy<br>
<a href="mailto:giovanni.cantele@spin.cnr.it"
style="color:rgb(17,85,204)" target="_blank"
moz-do-not-send="true">e-mail:
giovanni.cantele@spin.cnr.it</a><br>
Phone: +39 081 676910<br>
Skype contact: giocan74<br>
<br>
ResearcherID: <a
href="http://www.researcherid.com/rid/A-1951-2009"
style="color:rgb(17,85,204)" target="_blank"
moz-do-not-send="true" class="moz-txt-link-freetext">http://www.researcherid.com/rid/A-1951-2009</a><br>
Web page: </span><a
href="https://sites.google.com/view/giovanni-cantele/home"
style="color:rgb(17,85,204)" target="_blank"
moz-do-not-send="true" class="moz-txt-link-freetext">https://sites.google.com/view/giovanni-cantele/home</a><br>
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<div class="gmail_quote gmail_quote_container">
<div dir="ltr" class="gmail_attr">Il giorno gio 29 mag 2025 alle
ore 23:52 AMOGH A . <<a
href="mailto:f20220890@hyderabad.bits-pilani.ac.in"
moz-do-not-send="true" class="moz-txt-link-freetext">f20220890@hyderabad.bits-pilani.ac.in</a>>
ha scritto:<br>
</div>
<blockquote class="gmail_quote"
style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div dir="ltr">
<div dir="ltr">
<div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif">Hello folks!</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif"><br>
</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif">I am having 3
problems with MoSe2.</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif">
<ol>
<li>I got the correct electronic band structure of
the MoSe2 unitcell, but when I try it for the
supercell (3x3 or 5x5), or some doped version, the
band plots look very "busy" and messed up (plots
attached). The direct bandgap at the K-point also
disappears. Nothing except <i>nat</i> and <i>atomic_positions</i> changed
in the input files. Is this typical of
large atomic systems? Is there a way to reduce the
"business"? I read about "unfolding" the Brillouin
zone but not too sure how it might help.</li>
<li>
<div class="gmail_default">I am trying to get the
phonon dispersion and PhDOS for the monolayer. I
am working with the un-relaxed unitcell having 3
atoms. I have been getting frequencies from
-3000 cm^-1 to 2000 cm^-1 (plots attached). Can
someone please tell me why I am getting such
large negative frequencies and how to get the
correct plot? I tried changing almost all
parameters in the input file to no avail.</div>
</li>
<li>
<div class="gmail_default">MoSe2.dyn(i) files for
some values of i, will have a string of
asterisks in place of a few numbers which gives
me a Fortran compiler error when I run q2r.x (QE
version 7.4.1). Thus far, I have been going to
the files, finding the asterisks where numbers <i>should</i> be,
and replacing them with 0.000. Any idea why that
happens (and how to fix it)?</div>
</li>
</ol>
<div>Thanks for your help in advance!</div>
<div>
<div class="gmail_default">Amogh A</div>
<div class="gmail_default">Undergrad student at BITS
Pilani, Hyderabad.</div>
</div>
<div class="gmail_default"><br>
</div>
</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif">(GitHub and
Google Drive links)</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif"><a
href="https://github.com/AmoghA4/Quantum-ESPRESSO"
target="_blank" moz-do-not-send="true">Github rep
containing all the input files.</a><br>
</div>
</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif"><a
href="https://drive.google.com/file/d/1C_7zDhHir8eiHbNe2YyJhbrPSw7mSYjs/view?usp=sharing"
target="_blank" moz-do-not-send="true">Unitcell band
structure</a> v.s. <a
href="https://drive.google.com/file/d/1Dfxv2r9vj9FsLcI3cq3QVpkys2_ePyii/view?usp=sharing"
target="_blank" moz-do-not-send="true">supercell band
structure</a> and <a
href="https://drive.google.com/file/d/1XtAbmdGoNS8EklQ6GuJADlz6fLZREHmP/view?usp=sharing"
target="_blank" moz-do-not-send="true">oxygen-doped</a>.<br>
</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif"><a
href="https://drive.google.com/file/d/1plCzIl364o0ZEIkLxQujZ3Sg7hZpRHwU/view?usp=sharing"
target="_blank" moz-do-not-send="true">Phonon
dispersion plot I got</a> v.s. <a
href="https://www.researchgate.net/figure/The-phonon-dispersion-of-monolayer-a-MoS2-b-MoSe2-b-and-c-MoTe2-Non-negative_fig3_377939317"
target="_blank" moz-do-not-send="true">the correct one</a>.</div>
<div class="gmail_default"
style="font-family:tahoma,sans-serif"><br>
</div>
<div>
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