[Pw_forum] Questions on ibrav in pw.x

[Axel Kohlmeyer]

On Tue, May 28, 2013 at 9:57 AM, Yantao Wu <ywu at g.hmc.edu> wrote:
> Dear QE,
>
> I'm wondering what ibrav means in pw.x. My confusion is that if you can
> specify atomic positions freely, isn't the structure of the solid already
> implied by the positions? So why do we need the extra variable of ibrav. Or
> does ibrav carries more meaning than just the initial lattice structure of
> the solid?
>
> Another related question: When I ran vc-relax, I specified ibrav = 2 (fcc)
> and the atomic positions as follows:
>
> ATOMIC_POSITIONS {alat}
> Al   0.0      0.5     0.5
> Al   0.5      0.0     0.0
> Al   0.25     0.25    0.25
> Al   0.0      0.5     0.0
> Ti   0.75     0.75    0.75
> Ti   0.5      0.0     0.5
> Cr   0.0      0.0     0.0
> Cr   0.75     0.75    0.25
> Fe   0.5      0.5     0.0
> Fe   0.75     0.25    0.25
> Co   0.0      0.0     0.5
> Co   0.25     0.75    0.25
> Co   0.75     0.25    0.75
> Ni   0.25     0.75    0.75
> Ni   0.25     0.25    0.75
> Ni   0.5      0.5     0.5
>
> QE erred and complained that atom #1 and atom #6 overlap, which they
> apparently don't from the first sight. When I set ibrav = 1 (sc) and kept
> the same atomic positions, the complaint disappeared. So does anyone know
> why did the error occur?

because of the difference between the primitive cell and the
conventional cell. sc and fcc have the same conventional cell, but the
primitive fcc cell contains only a quarter of the atoms. obviously you
entered atoms in the conventional cell.
to be more explicit. the primitive cell of, say, an fcc metal contains
just one atom (one corner) and the basis vectors are not orthogonal,
whereas in the conventional cell, the basis vectors are orthogonal and
you have 4 atoms (one corner and 3 faces).

Q-E expects that you enter coordinates in the primitive cell only.
yet, since the sc primitive cell is identical to its conventional
cell, you can use an sc lattice to enter an fcc lattice geometry in
the conventional cell.

look it up in a crystallography text book (and on the web) and you'll see.

axel.

>
> I appreciate your replies with all my gratitude.
>
> Yantao "ignorant student" Wu
>
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--
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
International Centre for Theoretical Physics, Trieste. Italy.