Dear Lorenzo, thanks for your answer. I still have some doubts; when you say that
'In the limit where the {q} grid contains only the Gamma point, than each k-point exchanges only with itself (not with Gamma!).' it means that the q+k grid will end up to be the k grid. But then, 'In the opposite limit where the {q} grid has the same spacing as the {k} one (which may include a shift) then the {k} grid becomes equivalent to the {k+q} one, for every k. I.e. the {k} grid and any {k+q} grid are just shifted and re-indexed w.r.t each other. ' which is the difference with this second case? I have thought that when q grid and the k grid have the same spacing, the number of k+q points is larger than that of the k points, so they could not be equivalent. As you see, I am a beginner and a bit confused about this topic, do you have any paper to recommend me? Thank you very much, regards Valentina Il 04/15/2013 01:08 PM, Lorenzo Paulatto ha scritto: > On 04/15/2013 12:23 PM, "Valentina Dellac? C.R.F. S.C.p.A." wrote: >> Hi, >> I am having some doubts concerning nqx1,2,3 and K points grid when >> using hybrid functionals. As I understand, please let me know if I am >> wrong, there is no specific rule in how to choose the q point grid, >> for a given k point grid. The advice is to choose them to be the >> same, in order to avoid convergence issues. My question now is, since >> I read in hybrid functional README, that a shift in the q point grid >> is not implemented, can I pick a shifted k-point grid and a non >> shifted q point grid? Would it be better to pick two unshifted grids? > > Dear Valentina, > the grids are {k} and {k+q}, where the {q} grid is always > Gamma-centered. In other words, for each k point there is a > corresponding {k+q} grid centered around it. > > There is the additional constraint that each k+q point (for every k > and q) must be related to one of the initial k points by a G vector of > the reciprocal lattice and eventually a symmetry operation. I think > this condition should not cause any problem if the {k} grid is > shifted, but I'm not 100% sure. > > In the limit where the {q} grid contains only the Gamma point, than > each k-point exchanges only with itself (not with Gamma!). In the > opposite limit where the {q} grid has the same spacing as the {k} one > (which may include a shift) then the {k} grid becomes equivalent to > the {k+q} one, for every k. I.e. the {k} grid and any {k+q} grid are > just shifted and re-indexed w.r.t each other. > > I hope this helps, it is a bit confusing but it makes sense eventually. > > bests > > -- > Dr. Lorenzo Paulatto > IdR @ IMPMC -- CNRS& Universit? Paris 6 > phone:+33 (0)1 44275 084 / skype: paulatz > www:http://www-int.impmc.upmc.fr/~paulatto/ > mail: 23-24/4?16 Bo?te courrier 115, 4 place Jussieu 75252 Paris C?dex 5 -- -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20130415/cb62fc9b/attachment.html -------------- next part -------------- A non-text attachment was scrubbed... Name: firma 2013.jpg Type: image/jpeg Size: 19579 bytes Desc: not available Url : http://pwscf.org/pipermail/pw_forum/attachments/20130415/cb62fc9b/attachment.jpg