[Pw_forum] Questions on ibrav in pw.x

[mohnish pandey]

Hi Yantao,

A crystal is defined as lattice + basis. A bulk crystal has an infinite
size (infinite is a good approximation for the bulk description) and made
of identical units (which is one unitcell). So you need to know the
geometry of the unitcell (which is lattice) and what is it composed of
(which is basis), which in the input description is atomic positions. So
you need both the lattice type and atomic positions. This link will be
useful for you, you need to understand a bit of crystallography, that will
help you to get rid of the error you are getting.
http://www.vlab.msi.umn.edu/events/download/tutorial_wyckoff.pdf


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?
>
> I appreciate your replies with all my gratitude.
>
> Yantao "ignorant student" Wu
>
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-- 
Regards,
MOHNISH,
-----------------------------------------------------------------
Mohnish Pandey,
PhD Student,
Center for Atomic Scale Materials Design,
Department of Physics,
Technical University of Denmark
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