<div dir="ltr">Dear Giuseppe,<div><br></div><div>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.</div>
<div><br></div><div><div> &CONTROL</div><div> calculation = 'scf' ,</div><div> restart_mode = 'from_scratch' ,</div><div> outdir = '/home/matteo/qe/tmp/' ,</div>
<div> pseudo_dir = '/home/matteo/qe/espresso-5.0.2/pseudo/' ,</div><div> prefix = 'mgo_hse06' ,</div><div> verbosity = 'default' ,</div><div>
tstress = .true. ,</div><div> tprnfor = .true. ,</div><div> /</div><div> &SYSTEM</div><div> ibrav = 2,</div><div> celldm(1) = 7.9594,</div>
<div> nat = 2,</div><div> ntyp = 2,</div><div> ecutwfc = 100.0 ,</div><div><span class="" style="white-space:pre"> </span>nbnd = 8 ,</div>
<div> input_dft = 'hse', nqx1 = 2, nqx2 = 2, nqx3 = 2,</div><div> x_gamma_extrapolation = .true.,</div><div> exxdiv_treatment = 'gygi-baldereschi',</div><div> /</div><div>
&ELECTRONS</div>
<div> conv_thr = 1.0d-9 ,</div><div> mixing_beta = 0.7 ,</div><div> /</div><div> &IONS</div><div> ion_dynamics = 'bfgs' ,</div><div> /</div><div>ATOMIC_SPECIES</div>
<div> Mg 24.30500 Mg.pbe-hgh.UPF</div><div> O 15.99940 O.pbe-mt.UPF</div><div>ATOMIC_POSITIONS alat </div><div> Mg 0 0 0 </div><div> O 0.5 0.5 0.5</div><div>K_POINTS automatic </div><div>
4 4 4 0 0 0</div></div><div><br></div><div><br></div><div>Thank you for your help.</div><div><br></div><div>Matteo</div><div><br><div class="gmail_extra"><br><br><div class="gmail_quote">2013/11/29 Giuseppe Mattioli <span dir="ltr"><<a href="mailto:giuseppe.mattioli@ism.cnr.it" target="_blank">giuseppe.mattioli@ism.cnr.it</a>></span><br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><br>
Dear Matteo<br>
Which QE version are you using?<br>
I've clashed against similar issues, introduced in the late svn<br>
versions and (probably) corrected in the latest version, which were<br>
not present in the stable 5.0.2 version. If you are using the stable<br>
version, the problem should come out only in the case of strongly<br>
anisotropic (e.g., surface slabs with large vacuum) cells. Do you find<br>
it in bulk simulations (ZnO and TiO2, I suppose...:-))?. I've<br>
performed EXX simulations of these two guys, in the case of bulk as<br>
well as of surface cells. I did not fall into your dexx errors.<br>
HTH<br>
<br>
Giuseppe<br>
<br>
Giuseppe Mattioli<br>
ISM-CNR<br>
Italy<br>
<div><div class="h5"><br>
Quoting Matteo Gerosa <<a href="mailto:mgerosa88@gmail.com">mgerosa88@gmail.com</a>>:<br>
<br>
> Dear all,<br>
><br>
> I've been trying to calculate band gaps for some semiconductor oxides<br>
> using hybrid functionals (pbe0, hse, b3lyp).<br>
> I've learned the choice of the q-grid for computing the exchange energy<br>
> can be critical. Indeed in some cases I cannot achieve self-consistency<br>
> and an error message appears in the output file warning that the dexx<br>
> correction term becomes negative.<br>
> I guess this is related to the small-q divergence issue for some<br>
> particular q-grid patterns. In fact, if the k-grid is shifted away from<br>
> gamma (keeping the q-grid fixed) convergence is achieved. Unfortunately,<br>
> this doesn't allow me to compute the gap at the gamma point, which I'm<br>
> interested in. On the other hand, I also obtain convergence if I set<br>
> "x_gamma_extrapolation=.false.", but this induces large errors for a<br>
> small number of points in the q-grid.<br>
> How is it possible to solve the problem and, more importantly, predict<br>
> in which configurations of the q-grid could the problem arise? Is there<br>
> any way to shift the q-grid away from gamma?<br>
><br>
> Thanks for any help you can provide.<br>
><br>
> Best regards,<br>
><br>
> Matteo Gerosa<br>
><br>
> --<br>
> Matteo Gerosa<br>
> NanoLab - Micro and Nanostructured Materials Lab<br>
> Department of Energy, Politecnico di Milano<br>
> Via Ponzio 34/3 I-20133 Milano, Italy<br>
><br>
> tel. <a href="tel:%2B39%2002%202399%206364" value="+390223996364">+39 02 2399 6364</a><br>
> web: <a href="http://www.nanolab.polimi.it" target="_blank">www.nanolab.polimi.it</a><br>
><br>
><br>
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