[Wien] overestimated band gap by PBE

[Dr. K. C. Bhamu]

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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