Thank you very much Gabriele Sclauzero for your clear explanation. Now I understand what is happening in the band structure calculation.<br><br><div class="gmail_quote">On Wed, Apr 21, 2010 at 5:17 PM, Gabriele Sclauzero <span dir="ltr"><<a href="mailto:sclauzer@sissa.it">sclauzer@sissa.it</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div class="im"><br>
udayagiri sai babu wrote:<br>
> Dear all i am calculating electronic band structure of LaoFeP<br>
> superconductor. I have two questions regarding this.<br>
> As I have understood from the example01, I need to perform an "scf"<br>
> calculation followed by "bands" calculation.<br>
<br>
</div>That's right.<br>
<div class="im"><br>
> My question is<br>
> 1) Why cant we do bands calculation alone, as all we need is energy<br>
> eigen values at different k_points for all available bands. This can be<br>
> done by solving Kohn-Sham equations at those k points for all bands.<br>
<br>
</div>Because DFT is a self-consistent field (SCF) method! I hope this is clear to you, with all<br>
that this implies.<br>
<br>
Essentially, to get the self-consistent KS Hamiltonian you need the self-consistent charge<br>
density, which is defined as an integral in the 3D BZ. This integral is approximated with<br>
a sum over an appropriate set of k-points in the BZ (PWscf uses uniformly spaced regular<br>
grids, possibly shifted from the origin). Since you have to perform this kind of summation<br>
many times in a self consistent cycle (until converge is reached), you choose the minimal<br>
set of k-points which gives you the desired accuracy.<br>
<br>
Once you have the SCF Hamiltonian (only AFTER you got the SCF charge density), you can<br>
diagonalize it at any desired k-point to get the band structure along a path in reciprocal<br>
space. This path is not chosen in order to give a good sampling of the BZ (as instead the<br>
regular grid of above), but usually follows some high symmetry lines of the crystal.<br>
<div class="im"><br>
<br>
> 2) what data from scf calculation is used by bands calculation.<br>
<br>
</div>The self-consistent charge density is the key quantity. See also the replies from other<br>
people.<br>
<div class="im"><br>
><br>
> 3)when I did scf calculation for the following input file fermienergy is<br>
> not written in the output file. can somebody tell me if something is wrong?<br>
<br>
</div>Nothing wrong. The Fermi energy is not well defined for an insulator.<br>
<br>
HTH<br>
<br>
GS<br>
<div><div></div><div class="h5"><br>
><br>
> &control<br>
> calculation ='scf'<br>
> restart_mode='from_scratch',<br>
> outdir='/home/sai/laofep/',<br>
> prefix='laofep'<br>
> pseudo_dir='/home/sai/softwares/espresso-4.1.2/pseudo'<br>
> /<br>
> &system<br>
> ibrav=0,<br>
> celldm(1)=7.487,<br>
> celldm(3)=16.083,<br>
> nat=8,<br>
> ntyp=4,<br>
> ecutwfc=50,<br>
> ecutrho=500,<br>
> occupations='fixed',<br>
> nbnd=38,<br>
> /<br>
> &electrons<br>
> conv_thr=1.D-8,<br>
> diagonalization='david',<br>
> mixing_mode='plain',<br>
> mixing_beta= 0.7<br>
> /<br>
> ATOMIC_SPECIES<br>
> La 138.905 La.pw91-nsp-van.UPF<br>
> O 15.999 O.pw91-van_ak.UPF<br>
> P 30.973 P.pw91-van_ak.UPF<br>
> Fe 55.845 Fe.pw91-sp-van_ak.UPF<br>
> ATOMIC_POSITIONS (bohr)<br>
> La 0.000234206 3.687761102 10.487650296<br>
> La 3.687732709 0.000277249 5.739475052<br>
> O 0.000215849 0.000260237 8.115083977<br>
> O 3.687739522 3.687788174 8.115072590<br>
> Fe 0.000245844 0.000231960 0.091985046<br>
> Fe 3.687778452 3.687741841 0.091985975<br>
> P 3.687720269 0.000225890 14.089592313<br>
> P 0.000266430 3.687752524 2.133535171<br>
> K_POINTS {automatic}<br>
> 10 10 5 0 0 0<br>
> CELL_PARAMETERS<br>
> 0.985045304 0.000000280 -0.000002934<br>
> 0.000000280 0.985045750 0.000000396<br>
> -0.000004795 -0.000000455 2.142143981<br>
><br>
> --<br>
> U.Saibabu<br>
> PhD student,<br>
> Deformation mechanisms modeling group,<br>
> Materials engineering department,<br>
> IISc Bangalore,<br>
> India.<br>
><br>
><br>
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<br>
<br>
o ------------------------------------------------ o<br>
| Gabriele Sclauzero, PhD Student |<br>
</font><div class="im">| c/o: SISSA & CNR-INFM Democritos, |<br>
| via Beirut 2-4, 34014 Trieste (Italy) |<br>
</div><div class="im">| email: <a href="mailto:sclauzer@sissa.it">sclauzer@sissa.it</a> |<br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br>U.Saibabu<br>PhD student,<br>Deformation mechanisms modeling group,<br>Materials engineering department,<br>IISc Bangalore,<br>India.<br><br>