[Pw_forum] k points

[Axel Kohlmeyer]

On Tue, May 13, 2014 at 8:48 PM, Caroline <katesedate at gmail.com> wrote:

> 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?
>

​no.​



> 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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-- 
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.
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