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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>
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<p>Thanks for your attention, <br>
</p>
<p>Julien<br>
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