[Pw_forum] k points

[Caroline]

Hello Giovanni and Alex,
Thanks for your help. I went through some texts on k points again. So k
points have to cover all the lattice space in the unit cell to calculate
properties? Because Brilouin zones are defined all over the space, can we
say the larger the system (number of atoms) the more k points are needed to
obtain accurate properties?
Is this interpretation correct?


On 12 May 2014 10:05, Giovanni Cantele <giovanni.cantele at spin.cnr.it> wrote:

>
> On 12 May 2014, at 15:20, Caroline <katesedate at gmail.com> wrote:
>
> > Dear all,
> >
> > I am trying to understand the significance of k points in the input
> file. Can you please help me with the following questions:
> >
> > 1) How k points are selected? Dos it have to with the dimension of the
> crystal? Say for 2 dimension crystal only two lines in K point section is
> defined?
> >
> > 2) Or k points have to do with the numerical solution of integrals? I am
> confused how to define these values, and how they are important.
> >
> > 3) Is there a relation between number of k points and number of atoms in
> the lattice crystal?
> >
> >
> > I have read text books on this, and learned that k points need to
> selected in such a way that total energy is converged.
> >
> > Please help me with this.
> > Thanks a lot,
> > C.
> >
>
> no relation between the number of k-points and number of atoms. K-point
> grids are used to perform sums over the Brillouin zone and must be chosen
> such that the quantity you want to calculate is converged. For a 3D
> crystal you need a 3D grid, whereas if you lower the dimensionality of the
> crystal to 2D, 1D or 0D
> you will have 2D, 1D or 0D grids respectively (e.g. 4x4x1, 4x1x1, 1x1x1).
>
> You should better investigate the role of sums/integrals over the
> Brillouin zone using solid state textbooks, as well as try to follow the
> tutorials you can find on the Quantum-ESPRESSO
> web site, for example
> http://www.quantum-espresso.org/tutorials/shanghai-2013/ (but you can
> find many others at http://www.quantum-espresso.org/tutorials/).
>
> What is really important is to monitor how any property you are interested
> in (e.g. formation energy, phonon frequencies, bulk moduli) changes upon
> increasing the k-point grid. Also consider
> that different properties might require different grids to be converged.
> For example, charge density usually converges faster (with respect to
> k-point sampling) than phonon
> frequencies or optical properties.
>
> Giovanni
>
>
> PS please kindly provide an affiliation in a future message, as usually
> requested to the users of this forum.
>
> --
>
> 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
>
>
>
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-- 
Caroline G.
Universite de Sherbrooke
Canada
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