[Pw_forum] hybrid functionals: dexx<0

[Matteo Gerosa]

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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