[Wien] SCF convergence

[Peter Blaha]

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) 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
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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/
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