[Pw_forum] problem hybrid pseudopotential calculations

[Muhammad Adnan Saqlain]

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
>
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>



-- 
Best Regards
Muhammad Adnan Saqlain
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