[Pw_forum] hybrid functionals: dexx<0

[Giuseppe Mattioli]

Dear Matteo
Did you also try to play a bit with different exxdiv_treatment methods  
(with different ecutvcut cutoffs) and/or different PPs?
Ciao
Giuseppe

Quoting Matteo Gerosa <mgerosa88 at gmail.com>:

> Dear Giuseppe,
>
> thanks for your answer. I'm using the 5.0.2 version of QE plus 5.0.3
> patches. I'm running bulk calculations on MgO (rocksalt structure). The
> problem arises only for some q-grids (in one case, also when setting the
> q-grid equal to the k-grid), but I haven't understood why. And, as I said
> before, shifting the k-grid alone solves the problem. I attach here a
> sample input file.
>
>  &CONTROL
>                  calculation = 'scf' ,
>                 restart_mode = 'from_scratch' ,
>                       outdir = '/home/matteo/qe/tmp/' ,
>                   pseudo_dir = '/home/matteo/qe/espresso-5.0.2/pseudo/' ,
>                       prefix = 'mgo_hse06' ,
>                    verbosity = 'default' ,
>                      tstress = .true. ,
>                      tprnfor = .true. ,
>  /
>  &SYSTEM
>                            ibrav = 2,
>                      celldm(1) = 7.9594,
>                               nat = 2,
>                              ntyp = 2,
>                         ecutwfc = 100.0 ,
> nbnd = 8 ,
>                        input_dft = 'hse', nqx1 = 2, nqx2 = 2, nqx3 = 2,
>  x_gamma_extrapolation = .true.,
>             exxdiv_treatment = 'gygi-baldereschi',
>  /
>  &ELECTRONS
>                     conv_thr = 1.0d-9 ,
>                  mixing_beta = 0.7 ,
>  /
>  &IONS
>                 ion_dynamics = 'bfgs' ,
>  /
> ATOMIC_SPECIES
>    Mg   24.30500  Mg.pbe-hgh.UPF
>     O   15.99940  O.pbe-mt.UPF
> ATOMIC_POSITIONS alat
>    Mg      0    0    0
>     O      0.5  0.5  0.5
> K_POINTS automatic
>   4 4 4   0 0 0
>
>
> Thank you for your help.
>
> Matteo
>
>
>
> 2013/11/29 Giuseppe Mattioli <giuseppe.mattioli at ism.cnr.it>
>
>>
>> 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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>>
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>>     Giuseppe Mattioli
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-- 
********************************************************
- Article premier - Les hommes naissent et demeurent
libres et ègaux en droits. Les distinctions sociales
ne peuvent être fondèes que sur l'utilitè commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et
imprescriptibles de l'homme. Ces droits sont la libertè,
la propriètè, la sùretè et la rèsistance à l'oppression.
********************************************************

    Giuseppe Mattioli
    CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
    v. Salaria Km 29,300 - C.P. 10
    I 00015 - Monterotondo Stazione (RM)
    Tel + 39 06 90672836 - Fax +39 06 90672316
    E-mail: <giuseppe.mattioli at ism.cnr.it>