[Pw_forum] TiO2 001 Slab Convergence

[Aleksandra Oranskaia]

Hi Will,

1 
In your input I am confused with the combination of “relax” calculation and:
&CELL
cell_dynamics = 'bfgs',
press=0.1d0,
I do not know which calculation is eventually being done “relax” or “vc-relax”? If the second, I would doubt if it is correct to relax z-direction of the slab, especially accounting the pressure. 
I think that variable cell relaxation for the slab you might want to use cell_dofree = xy and treat z-parametr as a constant.

2
>From my little experience, increasing amount of vacuum to 20A might drastically improve the convergence, especially if 10A of vacuum is added bilaterally behind and above the slab (the slab atoms are put in the middle of the cell). This is connected with the dipole cancellation routine.

3
What might also decrease the convergence time is the decreasing your ecutrho cutoff, say to to around 5*ecutwfc. In pw.x manual it is suggested to use (8-12)*ecutwfc for UPF, but in papers people often use (5-6)*ecutwfc. Just to find the convergence grounds, you could start with 5*ecutwfc and after the initial relaxation switch to a larger cutoff.


It would be interesting to see the output. 


Anyway good luck with the convergence :)
Alex.
1st yr PhD student in KAUST, KSC, PSE
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