[QE-users] Si band structure using 8 atoms and SC lattice

[Giovanni Cantele]

Independently of whether one, both or no calculation(s) is (are) wrong, how do you judge whether or not a band structure is wrong?

A way to have a direct comparison between the two calculations is to compute the density of states of the two systems, since
the two density of states should be exactly the same (if both calculations are converged) , except for a scaling factor.

On the other hand, if you want to compare the band structures you should for example measure energy gaps. Because if you mean
that the second band structure looks different, this is indeed exactly what you expect. The second band structure is computed on
a larger unit cell (compare the unit cell volume  of the FCC and SC lattices), as such it has a smaller Brillouin zone and the
resulting bands are folded. Also, using the same special k-points does not give a direct term of comparison, since the high symmetry
points, usually used for plotting band structures, are different for different lattices. Last but not least, if in the second case you want to
plot the band structure along exactly the same path of the former, you should figure out how the k-points of the FCC Brillouin zone
read in terms of reciprocal lattico vectors of the FCC system.

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
            gcantele at gmail.com
Phone: +39 081 676910
Skype contact: giocan74
Web page: https://sites.google.com/view/giovanni-cantele

> On 19 Apr 2020, at 15:00, Mohamed Saleh <m.saleh at live.com> wrote:
> 
> Dear all,
> 
> I have successfully produced the band structure of bulk Si using FCC lattice with 2 atoms per basis. The relevant input sections are as follows:
> 
> &system
> ibrav=2, 
> celldm(1)=10.410909236,
> nat=2, 
> ntyp=1,
> nbnd=8
> /
> 
> ATOMIC_POSITIONS (alat)
> Si 0.00 0.00 0.00
> Si 0.25 0.25 0.25
> 
> However, when I try to get the same band structure using a SC unit cell that contain 8 Si atoms, I get completely wrong results. The relevant input sections are as follows:
> 
> &system
> ibrav=1, 
> celldm(1)=10.3557,
> nat=8, 
> ntyp=1,
> nbnd=32
> /
> 
> ATOMIC_POSITIONS (alat)
> Si 0 0 0
> Si 0 0.5 0.5
> Si 0.5 0.5 0
> Si 0.5 0 0.5
> Si 0.75 0.25 0.75
> Si 0.25 0.25 0.25
> Si 0.25 0.75 0.75
> Si 0.75 0.75 0.25
> 
> Both calculations use the same special k-points. The resulting band structures (the correct one and the wrong one) are attached. So what am I doing wrong?
> 
> Thanks in advance
> 
> 
> <Correct_BandStructure.png><Wrong_BandStructure.png>_______________________________________________
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