[Wien] SCF convergence

Luis Ogando lcodacal at gmail.com
Fri Sep 27 22:04:20 CEST 2013 

Dear Prof. Blaha (and others),

   Related to my previous messages, do you think that it would be useful to
change the mBJ parameters in order to get the experimental gap for the bulk
wurtzite InP with RKmax=5 and transfer them to the supercell keeping
RKmax=5 ?
   It would not be the most elegant solution, but it could be a solution.
   All the best,
                    Luis



2013/9/26 Luis Ogando <lcodacal at gmail.com>

> 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/**
>> theochem/ <http://info.tuwien.ac.at/theochem/>
>> ------------------------------**------------------------------**
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>
>
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