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: [email protected] [email protected] 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 <[email protected]> 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>_______________________________________________ > Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso > <http://www.max-centre.eu/quantum-espresso>) > users mailing list [email protected] > <mailto:[email protected]> > https://lists.quantum-espresso.org/mailman/listinfo/users > <https://lists.quantum-espresso.org/mailman/listinfo/users>
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