Dear Pascal, What you found is related to the Brillouin zone folding. You can find more details in Roarld Hoffmann's book: Solids and surfaces. The band structure using the primitive cell is correct.
Cheers, Yun On Mon, Mar 18, 2013 at 9:05 PM, pascal boulet <pascal.boulet at univ-amu.fr>wrote: > Dear all, > > We are facing a problem with differences in electronic band structures > when we use primitive or conventional unit cells. > > The structure we are dealing with is Mg2Si for which the primitive UC is: > ATOMIC_POSITIONS crystal > Si 0.000000000 0.000000000 0.000000000 > Mg 0.250000000 0.250000000 0.250000000 > Mg 0.750000000 0.750000000 0.750000000 > > The cubic space group is 225, a=12.0421785 bohr and Mg2Si is FCC. > > The rest of the input is (I make it short): > K_POINTS crystal > 120 > 0.5000000000 0.2500000000 0.7500000000 1.0 > 0.5000000000 0.2600000000 0.7400000000 1.0 > 0.5000000000 0.2700000000 0.7300000000 1.0 > etc. > calculation = 'bands', > celldm(1) =12.0421785, > nat = 3, > ibrav = 2, > > > If we use this UC we get a indirect band gap (G-X), which is correct. If > we now use the conventional UC (12 atoms in the cube) we get a direct band > gap at Gamma. In this case, we are using the following input file : > calculation = 'bands', > celldm(1) = 1, > nat = 12, > ibrav = 0, > > K_POINTS crystal > 126 > .0 .0 .0 1. > .020 .020 .020 1. > .040 .040 .040 1. > .060 .060 .060 1. > .080 .080 .080 1. > .100 .100 .100 1. > .120 .120 .120 1. > etc. > > CELL_PARAMETERS > 12.03517200 0.0 0.0 > 0.0 12.03517200 0.0 > 0.0 0.0 12.03517200 > ATOMIC_POSITIONS crystal > Si 0.00000 0.00000 0.00000 > Si 0.50000 0.50000 0.00000 > Si 0.00000 0.50000 0.50000 > Si 0.50000 0.00000 0.50000 > Mg 0.25 0.25 0.25 > Mg 0.75 0.75 0.75 > Mg 0.25 0.25 0.75 > Mg 0.25 0.75 0.25 > Mg 0.75 0.25 0.25 > Mg 0.25 0.75 0.75 > Mg 0.75 0.25 0.75 > Mg 0.75 0.75 0.25 > > Could you please tell us what we are doing wrong in the second calculation? > > Thank you for your response. > > Best regards, > Pascal > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum > -- Dr. Yun Wang Research Fellow Centre for Clean Environment and Energy Griffith School of Environment Gold Coast Campus, Griffith University QLD 4222, Australia Tel:(61-7) 5552 8456 Fax:(61-7) 5552 8067 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20130319/7e1107f8/attachment.html
