[Pw_forum] bands calculation

[udayagiri sai babu]

Thank you very much Gabriele Sclauzero for your clear explanation. Now I
understand what is happening in the band structure calculation.

On Wed, Apr 21, 2010 at 5:17 PM, Gabriele Sclauzero <sclauzer at sissa.it>wrote:

>
> udayagiri sai babu wrote:
> > Dear all i am calculating electronic band structure of LaoFeP
> > superconductor. I have two questions regarding this.
> > As I have understood from the example01, I need to perform an "scf"
> > calculation followed by "bands" calculation.
>
> That's right.
>
> > My question is
> > 1) Why cant we do bands calculation alone, as all we need is energy
> > eigen values at different k_points for all available bands. This can be
> > done by solving Kohn-Sham equations at those k points for all bands.
>
> Because DFT is a self-consistent field (SCF) method! I hope this is clear
> to you, with all
> that this implies.
>
> Essentially, to get the self-consistent KS Hamiltonian you need the
> self-consistent charge
> density, which is defined as an integral in the 3D BZ. This integral is
> approximated with
> a sum over an appropriate set of k-points in the BZ (PWscf uses uniformly
> spaced regular
> grids, possibly shifted from the origin). Since you have to perform this
> kind of summation
> many times in a self consistent cycle (until converge is reached), you
> choose the minimal
> set of k-points which gives you the desired accuracy.
>
> Once you have the SCF Hamiltonian (only AFTER you got the SCF charge
> density), you can
> diagonalize it at any desired k-point to get the band structure along a
> path in reciprocal
> space. This path is not chosen in order to give a good sampling of the BZ
> (as instead the
> regular grid of above), but usually follows some high symmetry lines of the
> crystal.
>
>
> > 2) what data from scf calculation is used by bands calculation.
>
> The self-consistent charge density is the key quantity. See also the
> replies from other
> people.
>
> >
> > 3)when I did scf calculation for the following input file fermienergy is
> > not written in the output file. can somebody tell me if something is
> wrong?
>
> Nothing wrong. The Fermi energy is not well defined for an insulator.
>
> HTH
>
> GS
>
> >
> > &control
> > calculation ='scf'
> > restart_mode='from_scratch',
> > outdir='/home/sai/laofep/',
> > prefix='laofep'
> > pseudo_dir='/home/sai/softwares/espresso-4.1.2/pseudo'
> > /
> > &system
> > ibrav=0,
> > celldm(1)=7.487,
> > celldm(3)=16.083,
> > nat=8,
> > ntyp=4,
> > ecutwfc=50,
> > ecutrho=500,
> > occupations='fixed',
> > nbnd=38,
> > /
> > &electrons
> > conv_thr=1.D-8,
> > diagonalization='david',
> > mixing_mode='plain',
> > mixing_beta= 0.7
> > /
> > ATOMIC_SPECIES
> > La 138.905  La.pw91-nsp-van.UPF
> > O 15.999 O.pw91-van_ak.UPF
> > P 30.973 P.pw91-van_ak.UPF
> > Fe 55.845 Fe.pw91-sp-van_ak.UPF
> > ATOMIC_POSITIONS (bohr)
> > La 0.000234206 3.687761102 10.487650296
> > La 3.687732709 0.000277249 5.739475052
> > O 0.000215849 0.000260237 8.115083977
> > O 3.687739522 3.687788174 8.115072590
> > Fe 0.000245844 0.000231960 0.091985046
> > Fe 3.687778452 3.687741841 0.091985975
> > P 3.687720269 0.000225890 14.089592313
> > P 0.000266430 3.687752524 2.133535171
> > K_POINTS {automatic}
> > 10 10 5 0 0 0
> > CELL_PARAMETERS
> > 0.985045304 0.000000280 -0.000002934
> > 0.000000280 0.985045750 0.000000396
> > -0.000004795 -0.000000455 2.142143981
> >
> > --
> > U.Saibabu
> > PhD student,
> > Deformation mechanisms modeling group,
> > Materials engineering department,
> > IISc Bangalore,
> > India.
> >
> >
> > ------------------------------------------------------------------------
> >
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>
> --
>
>
> o ------------------------------------------------ o
> | Gabriele Sclauzero, PhD Student                  |
> | c/o:   SISSA & CNR-INFM Democritos,              |
> |        via Beirut 2-4, 34014 Trieste (Italy)     |
> | email: sclauzer at sissa.it                         |
> | phone: +39 040 3787 511                          |
> | skype: gurlonotturno                             |
> o ------------------------------------------------ o
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-- 
U.Saibabu
PhD student,
Deformation mechanisms modeling group,
Materials engineering department,
IISc Bangalore,
India.
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