[Wien] SCF convergence

Luis Ogando lcodacal at gmail.com
Thu Sep 26 16:01:15 CEST 2013 

Dear Prof. Blaha,

   Thank you for your comments.
   I have reasons to change the default parameters and I would like to
clarify some points:

1) All the structural parameters were taken from optimized WZ and ZB bulk
InP structures using the PBE potential.

2) The increase in the number o K-points I mentioned in my first message
was done only in the a-b (x-y) plane, because there is no sense in
increasing K-points along the huge c-axis of the supercell.

3) Some time ago, there was a message sent to the list saying that an
increase in the Gmax value could improve the calculation performance. I did
some tests in the simple WZ structure and I got a faster calculation with
Gmax=20. This was (more or less) the value cited in the message, but,
unfortunately, I do not remember details (the message is somewhere in my
mailbox).

4) As you noted, the gap is the critical quantity in my calculations and I
did the suggested RKmax tests some time ago. The main parameters in these
tests were (only Gmax and Kpoints are different from that employed in the
supercell):

   *Structure: WZ InP bulk
   *RMT: 2.50 (In) and 2.10 (P)
   *Emax: 5 (spin-orbit effects)
   *XC: 13 (PBE) + mBJ
   *core-valence separation energy: -6.0
   *GMAX: 12 (NOT 20 !!!)
   *Inequivalent Kpoints: 16
   *Calculation options: run_lapw -p -NI -so -ec 0.0001 -cc 0.0001 -i 150
-it

Results:
   RKmax = 5 => gap = 1.556 eV
   RKmax = 6 => gap = 1.467 eV
   RKmax = 7 => gap = 1.476 eV
   RKmax = 8 => gap = 1.486 eV
   RKmax = 9 => gap = 1.493 eV   ( the experimental value is 1.49 eV ;  Appl.
Phys. Lett. 91, 263104 (2007) )
   RKmax = 10 => gap = 1.498 eV

   The difference between the calculated gaps for WZ and ZB bulk structures
is around 98 meV, but I doubt that the 3 ZB layers will reproduce the bulk
gap, so the difference between phases in the supercell must be lower than
98 meV. This is the reason for using RKmax = 9, I would like to get the
best gap value for the WZ region.

   In your message, you commented that the convergence may be disturbed by
ghostbands. Do you believe that the -in1new option can help ?

   I would strongly appreciate any comment / suggestion !
   Thank you again,
                             Luis



2013/9/26 Peter Blaha <pblaha at theochem.tuwien.ac.at>

> Reduce RKmax. It is ridiculous to start such a calculation with RKmax=9.
> Probably convergence was spoiled by the occurrence of "ghostbands".
>
> To do it right:
>
> Do a simple wurzite structure.
>
> Start with RKmax=5.5 (or even 5.0 )  (see http://www.wien2k.at/reg_user/**
> faq/rkmax.html <http://www.wien2k.at/reg_user/faq/rkmax.html>) and do:
>
> run_lapw -fc 1 -cc 0.0001     check gap and forces.
> save wc_rkm5.5
> increase rkmax to 6.5 in case.in1c
> run_lapw -fc 1 -cc 0.0001    check again gap and forces
>
> any significant changes ? If not, you have found a reasonable rkmax (5.5)
> to go to the "large calculation".
>
> For a BIG calculation ALWAYS start with low convergence parameters (which,
> if possible, should be tested before on a small system).
> Once the BIG calculation has converged and is fully relaxed, you can still
> check your results by increasing RKmax and continuing, but NEVER start with
> such huge values.
>
>
> On 09/25/2013 04:22 PM, Luis Ogando wrote:
>
>> Dear Wien2k community,
>>
>>     We are trying do calculate the influence of zinc blend (ZB) stacking
>> faults on InP (sp semiconductor) wurtzite (WZ) systems using Wien2k 13.
>> Our first goal is to calculate the band gap change along the c-axis
>> (perpendicular to the interface between the two phases), similar to what
>> was done in J. Appl. Phys. 114, 033709 (2013) for two semi-infinite
>> structures.
>>     In our case, we use a supercell with three ZB cells along (111)
>> (hexagonal form) matched along "c" to a large sequence of WZ cells to
>> guarantee a minimum reproduction of a ZB environment in the stacking
>> fault region.
>>     We have done calculations for a system with 10 WZ cells, but the gap
>> did not change along the structure and we are now trying 15 WZ cells and
>> the 3 ZB ones (78 atoms in total) to isolate neighbour stacking faults.
>>     As the gap is our main property, we are using the mBJ potential, but
>> we are facing convergence problems at the first (PBE) SCF calculation.
>>     We would be glad if someone could give us any hint about how to
>> improve the PBE scf convergence.
>>     Here we give some relevant parameters of our calculation (please, do
>> not hesitate in asking others):
>>
>> *Species: "In" and "P"
>> *Number of atoms: 78
>> *RMT: 2.50 (In) and 2.10 (P)  (NN-DIST= 4.84768)
>> *RKmax: 9
>> *Emax: 5 (spin-orbit effects)
>> *XC: 13 (PBE)
>> *core-valence separation energy: -6.0
>> *GMAX: 20
>> *Inequivalent Kpoints: 12 increased to 24 after some iterations
>> *Mixing schema: MSR1 changed to PRATT with 0.15 mixing factor after some
>> iterations
>> *Calculation options: run_lapw -p -NI -ec 0.0001 -cc 0.0001 -i 150 -it
>> ** We also attached the energy (Energy.dat) and charge (Charge.dat)
>> convergence evolution.
>>
>>     Many thanks in advance.
>>     All the best,
>>                      Luis
>>
>>
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>>
> --
>
>                                       P.Blaha
> ------------------------------**------------------------------**
> --------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
> Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/**
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