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 >> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://pwscf.org/mailman/listinfo/pw_forum >> > > > > -- > Caroline G. > Universite de Sherbrooke > Canada > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum > -- 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. -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140513/a09bad54/attachment.html
