[Wien] overestimated band gap by PBE

[Peter Blaha]

You cannot compare total energies when the calculations have different RMTs.

On 11/21/2016 01:25 PM, Dr. K. C. Bhamu wrote:
> 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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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
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