[Pw_forum] about band gap

[Giovanni Cantele]

Ezad Shojaee wrote:

> Hi everyone
> i am trying to obtain the band gap of TiO2(Anatase) and i did not have 
> better than about 9ev ( instead of  3.2 ! )  i have tried many paths 
> in the BZ including high-symmetry points & also i tried { automatic } 
> k_points  but it did not help.
> any suggestion will be appreciated
> this in my input file for band structure
>
> &control
>    calculation='nscf'
>    prefix='TiO2',
>    pseudo_dir = '/root/pseudo/',
>    outdir='/root/tmp/'
> /
> &system
>    ibrav= 7,  celldm(1) =7.155,   celldm(3) =2.5136,
>    nat=  6, ntyp= 2,
>    ecutwfc =50.0,
>    nbnd = 30,
> /
> &electrons
> /
> ATOMIC_SPECIES
> Ti  47.867     Ti.vdb.UPF
> O   15.9994    O.vdb.UPF
> ATOMIC_POSITIONS
> Ti  0.12500000 -0.12500000 -0.25000000
> Ti -0.12500000  0.12500000  0.25000000
> O   0.08159975  0.33159975 -0.16319950
> O  -0.08159975 -0.33159975  0.16319950
> O   0.33159975  0.08159975  0.33680050
> O  -0.33159975 -0.08159975 -0.33680050
> K_POINTS { automatic }
> 9  9  9   0 0 0
>
If you try to visualize your structure, it does not look really as an
anatase TiO2 crystal.
I think that you have two problems with you input file.
The anatase structure should be as follows:

Primitive vectors
a(1) =   3.78500000  0.00000000  0.00000000
a(2) =   0.00000000  3.78500000  0.00000000
a(3) =   1.89250000  1.89250000  4.75700000


Basis Vectors:
Atom    Lattice Coordinates                Cartesian Coordinates

Ti  0.12500000 -0.12500000 -0.25000000     0.00000000 -0.94625000 -1.18925000
Ti -0.12500000  0.12500000  0.25000000     0.00000000  0.94625000  1.18925000
O   0.08159975  0.33159975 -0.16319950     0.00000000  0.94625000 -0.77634000
O  -0.08159975 -0.33159975  0.16319950     0.00000000 -0.94625000  0.77634000
O   0.33159975  0.08159975  0.33680050     1.89250000  0.94625000  1.60216000
O  -0.33159975 -0.08159975 -0.33680050    -1.89250000 -0.94625000 -1.60216000


- 1st problem, from Doc/INPUT_PW:
ATOMIC_POSITIONS { alat | bohr | crystal | angstrom }

   alat    : atomic positions are in units of alat (default)
   bohr    : atomic positions are in a.u.
   crystal : atomic positions are in crystal coordinates (see below)
   angstrom: atomic positions are in A

It seems that you specified your positions in lattice (crystal) coordinates
without specifying {crystal} after ATOMIC_POSITIONS
The code will understand that those positions are in units of alat
(celldm(1))

- 2d problem, from Doc/INPUT_PW:

 body centered tetragonal (i)
  ================================
     a1 = (a/2)(1,-1,c/a),  a2 = (a/2)(1,1,c/a),  a3 = (a/2)(-1,-1,c/a).

This is the choice that PWSCF makes for the lattice vectors (you can 
check it in
your output,  they are explicitly written). It could be that if you 
specified the crystal
coordinates referred to a different choice of the crystal axes, the 
atoms are
not placed where they should.
So, check your axes and try to change the crystal coordinates 
accordingly or,
more simply, try to use Cartesian coordinates (listed above, in 
Angstrom). I tried
this last choice and it looks much better. If everything works well, you can
easily revert back to crystal coordinates reading them from the output


Hope this helps,

Giovanni


-- 



Dr. Giovanni Cantele
Coherentia CNR-INFM and Dipartimento di Scienze Fisiche
Universita' di Napoli "Federico II"
Complesso Universitario di Monte S. Angelo - Ed. G
Via Cintia, I-80126, Napoli, Italy
Phone: +39 081 676910
Fax:   +39 081 676346
E-mail: Giovanni.Cantele at na.infn.it
Web: http://people.na.infn.it/~cantele