On Jan 8, 2012, at 6:51 PM, Paolo Giannozzi wrote: just a few more more words
> On Jan 8, 2012, at 17:43 , Caloma Trumica wrote: > >> why do we need to have such an additional setting of q points >> rather than just using k points mesh for both? > > because they refer to different quantities and have different > convergence criteria. The "k-point" grid is used to perform the > integral over the Brillouin Zone; ... for electronic properties (squared Bloch states -> charge density, 1-electron energies -> sum thereof, etc.) > the "q-point" grid is used to > perform integrals over phonon wave vectors, ... which is also a sum over the Brillouin zone, but of quantities belonging to a different class of properties, which usually display different convergence properties (as pointed out by Paolo) > or to obtain > Interatomic Force Constants in real space. as above ... Also, q-point meshes usually include the gamma point (both because technically they often appear as differences between "k-points" q=k2-k1, hence if k2 and k1 both belong to a same mesh, q can be equal to zero; and also because one is often interested in zone-center phonons per se); k-points, instead, are often offset from the origin, because the sum over points of a regular grid of a periodic function converges slightly better as a function of the number of points if the grid is offset (this the somewhat obscure logics behind the construction of the Monkhorst-Pack sets of "special points"). SB --- Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype) La morale est une logique de l'action comme la logique est une morale de la pens?e - Jean Piaget -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20120108/b615806a/attachment.htm
