The total energy is not variational with respect to the number of
k-points, i.e. a lower total energy does not mean a better calculation
when you compare 2 k-meshes. Clearly the 1000k calculation should be better.
PS: The size of the basis set is variational: a larger RKmax has to give
you a lower energy.
PPS: As mentioned before, the GAP without spin-orbit is a meaningless
number and you should NEVER say that the calculation which gives for the
wrong reason the experimental gap is the "better" calculation.
Am 23.11.2016 um 13:15 schrieb Dr. K. C. Bhamu:
Dear Prof Peter,
Please comment of my query when you get time: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 *0.1 *-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)
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.
Results:
1. GAP 1.687 eV,FER: 0.1030934295, ENE: -339059.11079128
2. GAP 1.777 eV, FER: 0.1004906191, ENE: -339059.12432403
You see that band gap is in the error of /+- 0.01eV.
*[* next i tried with
1. with 1000 k, div: ( 11 7 11) and "min -j 'run_lapw -p -I -NI -i 120
-ec 0.1 _-cc_ 0.0001 -fc 1 >> second -ec is replaced by -cc
2. with 400 k, div: ( 8 5 8) and min -j 'run_lapw -p -I -NI -i
120 -ec 0.0001 _-cc _ 0.0005 -fc 2' ***>> second -ec is replaced by
-cc*
*
*and nothing happened scf immediately got exit and the structure was
already optimized]*
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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Inst.Materials Chemistry
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