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
<[email protected] <mailto:[email protected]>> 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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