[Pw_forum] fermi energy shift for different kmesh

Giovanni Cantele giovanni.cantele at spin.cnr.it
Wed Jun 12 10:44:22 CEST 2013

On Jun 12, 2013, at 9:38 AM, Yue-Wen Fang wrote:

> i have two urgent problems. 
> One is that, when i make scf calculation of LAO/STO superlattice for different kmesh, they got different fermi levers as well as the total energy. 
> below is some results. 
> 12*8*1 TOTEN = -348.979566 eV E-fermi : 4.7215 
> 12*8*2 TOTEN = -348.98343746 eV E-fermi : 4.7500 
> 12*8*4 TOTEN = -347.52711583 eV E-fermi : 7.3118 
> it's very strange they are so different, so i cannot belive in them. 
> The other question is that, after scf calculation on 12*8*2, i make a dos calculation, and find that the fermi energy is shifted from 4.7500 in scf to 7.4184 in dos. i also ploted the density of state in scf and dos calculation, former states insulator and latter is a metal phase. so ridiculous. 
> -- 
> ------------------------------------------------------------------------------------------------------------
> Yue-Wen FANG, PhD candidate
> Key Laboratory of Polar Materials and Devices, Ministry of Education
> East China Normal University 
> I will persist until I succeed!

As you write in your signature ("I will persist until I succeed") maybe you should persist in trying to understanding why QE is giving you right answers to
wrong questions. I mean that, provided your input has not syntax errors, QE will always output a total energy or a Fermi energy. Whether or not those
energies are the right ones and have a physical meaning depends on how you are performing the calculation.

I cannot say so much if you do not provide an input, but, based on your data:

1) your k meshes differ by the value of the number of k-points along the z direction (12x8x1, 12x8x2, 12x8x4). Any quantity you compute would be independent of this number
**ONLY IF** you expect that NO dispersion occurs along the z direction. When no dispersion occurs along the z-direction? If you are really dealing with
a superlattice, let's say an A-B-A-B-A-B-.... stacking, you supercell contains an A-B unit. Now, such unit is periodically repeated along the z-direction,
according to the Bravais lattice you specify in input. Let's call c the lattice parameter along the z direction. The distance between two periodic replicas
of the A subunit is c. If c is sufficiently large, then you results might appear strange and the explanation should be found elsewhere. But if c is just few A,
than there is interaction between periodic replicas along the z direction. As such, dispersion would occur, and sampling the BZ along the Z direction with
just one k-point would provide not-converged results.

2) DOS calculations usually require k-meshes larger or much larger that those you need to converge the charge density in you scf calculation. If you look,
for example, to espresso-5.0.2/PP/examples/example02, where the DOS of Ni is computed, the scf calculation is performed with 60 k points, but the DOS using
a 12x12x12 grid. How are you performing your DOS calculations? And what does it mean "density of state in scf and dos calculation"? The DOS is computed only
after an "nscf" calculation following the scf.

3) If your system is metallic, than you need also to specify a smearing to help the system in converging with respect to the k-point grid. Also this parameter must be
carefully checked.

I suggest you to first check the convergence of your scf calculation with respect the k-point grid and any other calculation parameter you think might be relevant.
Then perform the DOS calculation, according to the right procedure. With just some patience, you can easily change your results from ridiculous to reasonable.



**** PLEASE NOTICE THE NEW E-MAIL ADDRESS: giovanni.cantele at spin.cnr.it

Giovanni Cantele, PhD
c/o Dipartimento di Scienze Fisiche
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele

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