[QE-users] forcing cubic symmetry for a slab

[Julien Barbaud]

Dear users,


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.

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.

My question boils down to this:*Is there a good method force a slab to 
remain cubic, even though it has this additional freedom?*

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)

Let me know if you need any more information!


Thanks for your attention,

Julien

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