[Pw_forum] hybrid functionals: dexx<0

[Giuseppe Mattioli]

Dear Matteo
Which QE version are you using?
I've clashed against similar issues, introduced in the late svn  
versions and (probably) corrected in the latest version, which were  
not present in the stable 5.0.2 version. If you are using the stable  
version, the problem should come out only in the case of strongly  
anisotropic (e.g., surface slabs with large vacuum) cells. Do you find  
it in bulk simulations (ZnO and TiO2, I suppose...:-))?. I've  
performed EXX simulations of these two guys, in the case of bulk as  
well as of surface cells. I did not fall into your dexx errors.
HTH

Giuseppe

Giuseppe Mattioli
ISM-CNR
Italy

Quoting Matteo Gerosa <mgerosa88 at gmail.com>:

> Dear all,
>
> I've been trying to calculate band gaps for some semiconductor oxides
> using hybrid functionals (pbe0, hse, b3lyp).
> I've learned the choice of the q-grid for computing the exchange energy
> can be critical. Indeed in some cases I cannot achieve self-consistency
> and an error message appears in the output file warning that the dexx
> correction term becomes negative.
> I guess this is related to the small-q divergence issue for some
> particular q-grid patterns. In fact, if the k-grid is shifted away from
> gamma (keeping the q-grid fixed) convergence is achieved. Unfortunately,
> this doesn't allow me to compute the gap at the gamma point, which I'm
> interested in. On the other hand, I also obtain convergence if I set
> "x_gamma_extrapolation=.false.", but this induces large errors for a
> small number of points in the q-grid.
> How is it possible to solve the problem and, more importantly, predict
> in which configurations of the q-grid could the problem arise? Is there
> any way to shift the q-grid away from gamma?
>
> Thanks for any help you can provide.
>
> Best regards,
>
> Matteo Gerosa
>
> --
> Matteo Gerosa
> NanoLab - Micro and Nanostructured Materials Lab
> Department of Energy, Politecnico di Milano
> Via Ponzio 34/3 I-20133 Milano, Italy
>
> tel. +39 02 2399 6364
> web: www.nanolab.polimi.it
>
>
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    Giuseppe Mattioli
    CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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