[Pw_forum] Questions on ibrav in pw.x

[Yantao Wu]

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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