[Pw_forum] nscf calculations

[Giovanni Cantele]

On Mar 24, 2010, at 12:46 PM, Dimpy Sharma wrote:

> Hi there,
> 
> I have submitted my input file for nscf calcuation with 22 kpoints ( for the first brilloiun zone from kpoint from k=0 to 0.47) however in the output file of nscf , it calcuates the eigenvalues at k points 0,0.148,0.2960..3.1076 .Thus while plotting the band diagram my kvectir range becomes upto 3.1076. May I know why it happen? and to calculate the eigenvalues only on the given kpoints in th input file?
> 
> Thanks
> 
> Dimpy
> 
> Dimpy Sharma(Msc Nanotechnology
>               PhD)
> UCC
> Ireland
> 


possible issues:

i) if the number of k-points in the output is greater than the number of k-points you specify in the input, then the issue can be related
to symmetries. Unless you explicitly specify nosym = .true. (in nscf) the code always "complete" the k-point list in such a way it gets
compatible with the symmetry operations

ii) you are just "reading" your k-points in different units than you expect. Indeed, when the codes prints out the eigenvalues at each
k-point, the coordinates of the latter are tpiba, namely, 2 * pi / a (where a is the lattice constant along the first direct lattice axis).
Therefore, if you give, in the input
K_POINTS {crystal}
0.0    0.0    0.5    1.0
then if the system is not cubic ad c is the lattice parameter along the third direct lattice axis, the coordinates found in the output
will be 0.5 *a / c
I think that the band structure calculation requires not to keep the original "crystal units", because if you want to compute the band structure
along a given path in the Brilluoin zone, you want to keep the relative dimension of different segments (e.g. L-Gamma, Gamma-X) and for
that you cannot use a different unit along different directions (as it would occur in crystal coordinates in non-cubic systems).

giovanni

--

Giovanni Cantele, PhD
CNR-SPIN and 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 - Fax:   +39 081 676346
Skype contact: giocan74

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

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