Thanks a lot for your reply. Your inputs did clear my doubts to a large extent. I understand that I can get the band diagram using a 2-atom fcc crystal. However, the next band diagram which I wanted to obtain contained 1 atom of Li as impurity in the 64 atom bulk Si. This system, I think can not be broken down in a simpler system. So, what should I do in this case?
..... Thanks, Mitul Mundra Final Year Dual Degree Student Department of Chemical Engineering IIT Kanpur, India. On Thu, Jan 23, 2014 at 5:04 PM, Giovanni Cantele < giovanni.cantele at spin.cnr.it> wrote: > > On 23 Jan 2014, at 11:46, Mitul Mundra <mitulm at iitk.ac.in> wrote: > > Hello, > > I am a beginner in using Quantum Espresso. I have a 64 atom Si supercell > for which I am trying to plot band diagrams. I have gone through the > tutorial "PWSCF: First Steps" available at > http://www.fisica.uniud.it/~giannozz/QE-Tutorial/tutorial_pwscf_ex.pdf . > The problem I am facing is that when I run a scf calculation using 4*4*4 > k-points, my output file has energies of 128 bands for every k-point. For > example, > *k = 0.0000 0.0000 0.0000 ( 12797 PWs) bands (ev):* > > * -5.6509 -4.5410 -4.5410 -4.5410 -4.5410 -4.5410 -4.5410 > -3.5502* > * -3.5502 -3.5502 -3.5502 -3.5502 -3.5502 -3.5502 -3.5502 > -3.5502* > * -3.5502 -3.5502 -3.5502 -3.2434 -3.2434 -3.2434 -3.2434 > -1.3979* > * -1.3979 -1.3979 -1.3978 -1.3978 -1.3978 -1.2206 -1.2206 > -1.2206* > * -1.2206 -1.2206 -1.2206 -1.2205 -1.2205 -1.2205 -1.2205 > -1.2205* > * -1.2205 -0.5951 -0.5951 -0.5951 -0.5951 0.9797 0.9797 > 0.9797* > * 0.9797 0.9797 0.9797 0.9797 0.9797 0.9797 0.9797 > 0.9797* > * 0.9797 2.5385 2.5385 2.5385 2.5385 2.5385 2.5385 > 2.5385* > * 2.5385 2.5385 2.5385 2.5385 2.5385 2.6504 2.6504 > 2.6504* > * 2.6504 2.6504 2.6504 2.6504 2.6504 2.6504 2.6504 > 2.6504* > * 2.6504 3.0031 3.0031 3.0031 3.0031 3.0031 3.0031 > 3.5849* > * 3.5849 3.5849 3.5849 3.5849 3.5849 4.5713 4.5713 > 4.5713* > * 4.5713 4.5713 4.5713 4.5713 4.5713 4.5713 4.5713 > 4.5713* > * 4.5713 5.1567 5.1567 5.1567 5.1567 5.1568 5.1568 > 5.1568* > * 5.1568 5.1568 5.1568 5.1568 5.1568 5.2932 5.2932 > 5.2932* > * 5.2932 5.2932 5.2932 5.2932 5.2932 6.5240 6.5240 > 6.5240* > > On doing calculation = 'bands' and nbnd = 8, the output file contains > energies of only 8 bands for every k-point which are the lowermost bands. > *k = 0.0000 0.0000 0.0000 band energies (ev):* > > * -5.6509 -4.5410 -4.5410 -4.5410 -4.5410 -4.5410 -4.5410 > -3.5502* > Then I run a bands.x executable to genrate bands.dat. On using plotband.x, > I get the following. > > */opt/apps/espresso-5.0.1/bin/plotband.x* > *Input file > bands.dat* > *Reading 8 bands at 10 k-points* > *Range: -5.6510 -3.5500eV Emin, Emax > -6, -3* > > *high-symmetry point: 0.0000 0.0000 0.0000 x coordinate 0.0000* > *...* > > *high-symmetry point: -0.5000-0.5000-0.5000 x coordinate 0.0000* > *output file (xmgr) > si.bands.xmgr* > *bands in xmgr format written to file si.bands.xmgr > > > * > *output file (ps) > si.bands.ps <http://si.bands.ps/>* > *Efermi > -3.987 * > *deltaE, reference E (for tics) 1.0, -3.987* > *n= 2 3* > * 0.0000000E+00 0.0000000E+00* > * -5.651000 -5.581000 * > * NaN NaN * > .... > .... > .... > and more NaNs and my *si.bands.ps <http://si.bands.ps/>* file contains no > plot. > > I would be extremely thankful if someone could tell me where am I wrong > and what different can I do to obtain the band plot (4 valence bands and 4 > conduction bands). > > Thanks, > Mitul Mundra > Final Year Dual Degree Student > Department of Chemical Engineering > IIT Kanpur, India. > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum > > > > > If you want to obtain the ?textbook? band plot of silicon (4 valence bands > and 4 conduction bands) you CANNOT use a 64-atom supercell, but > the 2-atom fcc unit cell of the silicon crystal. > > Whatever lattice you are using, a 64-atom supercell contains 4*64 valence > electrons, corresponding to 2*64=128 occupied (valence) bands! > That means that, even though you?re describing the silicon crystal just > using a larger supercell (the textbook 2-atom fcc cell is the minimal choice > among a set of infinite equivalent choices of the unit cell), you get > EXACTLY the same band structure but described within the Brillouin zone > of the lattice you are considering. > > So, let?s suppose that you choose to replicate the Si fcc unit cell twice > in each direction, thus using a 2x2x2 supercell, the Brillouin zone of this > crystal is 1/2 x 1/2 x 1/2 = 1/8 smaller that the Brillouin zone of the > crystal as described with the minimal cell. > Because the electronic properties cannot depend on the choice of the unit > cell, the silicon bands get folded into the smaller Brillouin zone. > Plotting this band structure is possible, but of course you obtain many > more bands, which are more difficult to handle with than the simpler > Si band structure with only 4 valence bands. Therefore, the two band > structures are DIFFERENT even though EQUIVALENT. > > To get rid of this difficulty, if you really want to describe bulk Si with > your unit cell, you could firstly calculate the density of states (DOS), > Indeed, > being the DOS an integral over the Brillouin zone, the minimal and larger > unit cell produce the SAME DOS with only a proportionality factor > of difference (in my example above, the 2x2x2 cell will produce the 8 > times the DOS of the 1x1x1 unit cell). In this case comparing the two > results is more straightforward. > > 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 > Phone: +39 081 676910 > Skype contact: giocan74 > > ResearcherID: http://www.researcherid.com/rid/A-1951-2009 > Web page: http://people.na.infn.it/~cantele > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140123/fefd1889/attachment.html
