[QE-users] Simple cubic to fcc cubic

[Giovanni Cantele]

Let us suppose that we have a fcc crystal, with one atom per cell, at
(0,0,0), for the sake of simplicity.

The unit cell sides are in the form (a/2,a/2,0) (a=side of the cubic cell),
with unit cell volume a^3 / 4.

Now, let us move from fcc to sc unit cell.
Each of the 8 atoms at the corners is shared by 8 cubic cells, and thus we
should consider only one of these atoms.
On the other hand, atoms at the center of the cube faces are shared by two
unit cells, and count as 1/2. Since we have
6 faces, 3 out of these 6 atoms must be considered.
As a result, the sc unit cell will contain 4 atoms and a volume 4 times
larger, a^3 as you would expect.

To go back from sc to fcc you should therefore keep 1/4 of your atoms,
because those that are associated to the centers of three faces,
that should be explicitly considered in sc, now become equivalent (that is,
connected by a lattice vector) to other atoms in fcc.

The code correctly finds overlapped atoms, e.g. in the example of one atom
per unit cell, in sc you would have (0,0,0), (a/2,a/2,0), (a/2,0,a/2),
(0,a/2,a/2)
and the last three in fcc are obtained from (0,0,0) after a lattice
translation by (a/2,a/2,0), (a/2,0,a/2), (0,a/2,a/2), respectively.

Giovanni

-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it <giovanni.cantele at spin.cnr.it>
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: https://sites.google.com/view/giovanni-cantele/home


Il giorno mar 27 feb 2024 alle ore 11:14 Ms. Chandrika K. via users <
users at lists.quantum-espresso.org> ha scritto:

> Need help in understanding
>
>    1. I am currently working on Co3O4 structure,from literature it is
>    very evident that it belongs to space group Fd3m(cubic fcc).
>    2. I have referred to Co3O4 Fd3m cubic structure from materials
>    project it is represented as simple cubic structure but Fd3m space group
>    refers to cubic fcc.
>    3. As it is simple cubic ibrav =1,when i change ibrav =2(cubic fcc) i
>    am getting warning message
>    "WARNING: Atom 381 and atom 444 are very close !!!   Atom 444 deleted
>    !!!"
>    ATOMIC_POSITIONS (angstrom)
>
> I would like to know how to change the structure from simple cubic to fcc
> cubic
>
>
> chandrika yadav k
>
>
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