[Pw_forum] problem hybrid pseudopotential calculations
stefano de gironcoli
degironc at sissa.it
Sat Jul 23 15:48:11 CEST 2016
On 23/07/2016 14:23, Muhammad Adnan Saqlain wrote:
> Dear Stefano
> thanks for your response.
> Can you please guide me (1) Why vc-relax is not allowed in Q-E 5.4.0?
it is.
> (2) How would the band gap be effected if I relax the cell without
> hybrid scheme and then calculate band gap with hybrid one?
>
it would be the band gap of a different structure.
it would depend on how different the structure would be with a different
functional.
stefano
>
> On Sat, Jul 23, 2016 at 1:07 PM, stefano de gironcoli
> <degironc at sissa.it <mailto: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 <mailto: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
>>
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>>
>>
>> --
>> Best Regards
>> Muhammad Adnan Saqlain
>>
>>
>>
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> --
> Best Regards
> Muhammad Adnan Saqlain
>
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