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<p>Dear Julien,</p>
<p>A slab can never be cubic. At most it can be forced to remain
tetragonal. But I see that your perovskite contains formamidinium
cations. Even if you force the dimensions of the bulk cell to
remain cubic, the internal symmetry after relaxation will not be
cubic; those organic cations lack threefold symmetry, which is the
condition for a structure to be cubic by symmetry. This said, a
correct arrangement of the formamidinium ions might allow having
tetragonal symmetry, both in the bulk and in the slab.<br>
</p>
<p>José C. Conesa<br>
</p>
<div class="moz-cite-prefix">El 21/02/2020 a las 13:13, Julien
Barbaud escribió:<br>
</div>
<blockquote type="cite"
cite="mid:2684cf98-cd75-c9f9-e74f-51aab5415f5b@sjtu.edu.cn">
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<p>Dear users,</p>
<p><br>
</p>
<p>I am trying to simulate the surface of a perovskite. If I let
vc-relax do its job on my system, it ends up yielding a
monoclinic unit cell. However, the phase of interest for my
purposes has a cubic symmetry (actually becomes stable only at
at higher temperatures). When it comes to bulk calculations,
this is no problem, because we can easily constrain the system
to a cubic symmetry and let it relax (constrained vc-relax, or
Birch fit method). And I do get very good agreement with
experimental lattice constants.</p>
<p>When I try to simulate a slab of a few atomic layers, however,
the vacuum thickness left in the unit cell allows for one more
degree of freedom for the slab. This results in my atomic slab
"tilting" during the relax calculation, effectively mimicking
the monoclinic result (without the actual fictitious "slab cell"
changing, of course). I am afraid this description might not be
clear, so I attached an illustration of the problem I get after
a relax run on an initially cubic slab.</p>
<p>My question boils down to this:<b> Is there a good method force
a slab to remain cubic, even though it has this additional
freedom?</b></p>
<p>This sounds like a basic question, but I couldn't find an
answer so far. I thought of limiting the relaxation the the
z-axis only, but I believe that would not be satisfying for the
physics of my system (mostly because my material is made of
organic cations encaged in an inorganic framework, and the
orientation/rotation of those cations usually are relevant. If I
don't allow any movement in the x-y directions, I won't be able
to observe potential re-orientation of the cations at the
surface)</p>
<p>Let me know if you need any more information!</p>
<p><br>
</p>
<p>Thanks for your attention, <br>
</p>
<p>Julien<br>
</p>
<br>
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</blockquote>
<pre class="moz-signature" cols="72">--
José C. Conesa
Research Professor
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Campus de Cantoblanco
Tel. 915854766
Madrid, Spain</pre>
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