[Wien] overestimated band gap by PBE

[Xavier Rocquefelte]

Dear Bhamu

First of all, your band gap to be accurate need many kpoints when taken 
from the SCF file.

The best is to plot the band structure. Then you will have the 
fundamental band gap, not the optical band gap (usually larger).

In your case, you must also include the spin-orbit coupling to properly 
treat the lead states which define the band gap.

Including the spin-orbit coupling will reduce your band gap and thus you 
will reach the GGA underestimation ;)

Cheers

Xavier


Le 21/11/2016 à 13:25, Dr. K. C. Bhamu a écrit :
> Dear Prof. Peter,
>
> This is in the queue on my previous query regarding CH3NH3PbI3:
>
> I used two strategy and got different results:
>
>
> The experimental band gap is 1.67 eV for orthorhombic lead halide 
> perovskite.
>
> I ran two cases (with PBE);
> 1. with 1000 k, div: ( 11  7 11) and "min -j 'run_lapw -p -I -NI -i 
> 120 *-ec *0.00001 *-ec * 0.0001 -fc 1
> 2. with 400 k, div: (  8  5  8)   and   min -j 'run_lapw -p -I -NI -i 
> 120 *-ec * 0.0001 *-ec * 0.0005 -fc 2'
>
> -ec switch is used twice instead of ec and cc (it is by mistake)
>
>
> Results:
>
> 1. GAP 1.687 eV,    FER: 0.1030934295, ENE: -339059.11079128
> 2. GAP  1.777 eV,   FER: 0.1004906191, ENE: -339059.12432403
>
> In case 2 with less k-points and normal scf criteria (-ec 0.0001 -ec 
> 0.0005 -fc 2), I got minimized ENE than case 1. But the band gap is 
> good from case 1.
>
> You see that band gap is in the error of /+- 0.01eV.
>
>
> I did nothing special, just reduced rmt (5% and further rmt of "I" was 
> reduced by 0.02).
> Please correct me what is wrong here. Because usually PBE always 
> underestimates the experimental band gap.
>
> Kind regards
> Bhamu
>
>
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