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/20130415/2267270d/attachment.html