[QE-users] crystal definition

[Tone Kokalj]

On Sun, 2021-01-31 at 06:39 +0000, Husak Michal wrote:
> Forget this method.
> Get a CIF file for the structure you want to stady and
> use the space group based crystal definition ...

I completely disagree. 

What Valeria described is all correct. The question to answer is this:
why is the total energy with ibrav=3 twice smaller than that for
ibrav=0 with two atoms in the unit cell.

With ibrav=3, there is one atom per primitive bcc unit cell, whereas
with ibrav=0 the bcc crystal was described as primitive-cubic with two
atoms per unit-cell. In xcrysden these are called primitive vs
conventional cell mode. To understand the difference, load the ibrav=3
input in xcrysden and switch between the two (i.e. Display-->Primitive
Cell Mode vs. Display-->Conventional Cell Mode).

Best regards, Tone
-- 
Jožef Stefan Institute, Ljubljana, Slovenia