[Pw_forum] problem hybrid pseudopotential calculations

[stefano de gironcoli]

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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