[Pw_forum] problem hybrid pseudopotential calculations

[Muhammad Adnan Saqlain]

Dear Stefano
thanks for your response.
Can you please guide me (1) Why vc-relax is not allowed in  Q-E 5.4.0?
(2) How would the band gap be effected if I relax the cell without hybrid
scheme and then calculate band gap with hybrid one?


On Sat, Jul 23, 2016 at 1:07 PM, stefano de gironcoli <degironc at sissa.it>
wrote:

> dear Muhammad Adnan Saqlain,
>
>    gamma extrapolation is a way to deal with the ill defined value of the
> exchange integral between wfcs at k and k+q in the limit of q->0.
>    if you use a truncated coulomb interaction that limit is not ill
> defined.
>    no need to use the gamma_extrapolation option
>    best,
> stefano
>
> On 23/07/2016 10:00, Muhammad Adnan Saqlain wrote:
>
> Dear Prof Clavijo
> I am extremely sorry for my misleading title of the calculation. Actually,
> It was a cubic structure consisting of Cs, Pb and I. I deliberately did not
> put the atoms in the calculation. more over I do not know why the algorithm
> for cell movement was not copied it was however present in the input file.
>
> 1. If I try these two options, the
>
> vcut_spherical : appropriate for cubic and quasi-cubic supercells
> vcut_ws : appropriate for strongly anisotropic supercells, see also
>           ecutvcut.
>
> the program says like this
>
>
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>      Error in routine  system_checkin (1):
>       x_gamma_extrapolation cannot be used with vcut
>
>  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>
> I have no idea how to fix it. Can you suggest one??
>
> 2. I have now used Q-E 5.4.0, but this version does not allow vc-relax
> option with hybrid one. why is it so? How would the band gap be effected if
> I relax the cell without hybrid scheme and then calculate band gap with
> hybrid one?
>
> 3. About the ecutoff, I deliberately used small one as these are just
> calculations not the real one.
> 4. I have cubic system, may I try q-grid like 1*2*3 or it has to be
> symmetric one?
>  5. Can you suggest more about hybrid calculations?
>
>
> On Fri, Jul 22, 2016 at 10:09 PM, Josue Itsman Clavijo Penagos <
> jiclavijop at unal.edu.co> wrote:
>
>> Dear, Muhammad Adnan Saqlain, remember the user documentation for
>> *exxdiv_treatment*:
>>
>> Specific for EXX. It selects the kind of approach to be used
>> for treating the Coulomb potential divergencies at small q vectors.
>>
>> gygi-baldereschi : appropriate for cubic and *quasi-cubic supercells*
>> vcut_spherical : appropriate for cubic and quasi-cubic supercells
>> vcut_ws : appropriate for strongly anisotropic supercells, see also
>>           ecutvcut.
>> none : sets Coulomb potential at G,q=0 to 0.0 (required for GAU-PBE)
>>
>> *Besides that, *I have some observations and suggestions for you based
>> in the input file you send, so you can realize why does QE is failing when
>> trying to simulate your system:
>>
>> 1 - Check if you really need to apply smearing, since this technique is
>> intended to be useful for metallic systems, and Anatase is cleary not a
>> metallic system, in fact, is more a Wide-gap or Insulator -type system.
>>
>> 2 - You do want to perform a vc-relax calculation, but nothing are
>> specified in the cards &IONS and &CELL. There is not cell relaxing
>> algorithm specified (BFGS? MD?)
>>
>> 3 - The card &ATOMIC_POSITIONS shows nothing: No atoms, no cell to relax.
>>
>> 4 - In the card &SYSTEM , *ecutwfc = 40.* This, in my humble opinion, a
>> very poor cutoff for a serious calculation. Do a ecutwfc convergence test
>> before attempt your system.
>>
>> 5 - Please take in account that a hybrid calculation is VERY expensive in
>> terms of computational demand, so if you set up a vc-relax calculation
>> using hybrid PPS you must test if *nqx1 = 1, 2 or 3 really makes a
>> difference.*
>>
>> *6 - For some systems, input_dft=HSE is faster than **input_dft =
>> "PBE0". Check if that could be your case.*
>>
>> By the way, are you sure Anatase has a cubic crystal structure? Does
>> XCrysden show the correct structure for your input?
>>
>> *Best regards, *
>>
>>
>> Josué Clavijo, PhD.
>> Assistant Professor
>> Universidad Nacional de Colombia
>> Departamento de Química
>>
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://pwscf.org/mailman/listinfo/pw_forum
>>
>
>
>
> --
> Best Regards
> Muhammad Adnan Saqlain
>
>
>
> _______________________________________________
> Pw_forum mailing listPw_forum at pwscf.orghttp://pwscf.org/mailman/listinfo/pw_forum
>
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>



-- 
Best Regards
Muhammad Adnan Saqlain
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20160723/52fe3fc0/attachment.html>