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<p>Dear Bhamu</p>
<p>First of all, your band gap to be accurate need many kpoints when
taken from the SCF file. <br>
</p>
<p>The best is to plot the band structure. Then you will have the
fundamental band gap, not the optical band gap (usually larger). <br>
</p>
<p>In your case, you must also include the spin-orbit coupling to
properly treat the lead states which define the band gap. <br>
</p>
<p>Including the spin-orbit coupling will reduce your band gap and
thus you will reach the GGA underestimation ;) <br>
</p>
<p>Cheers</p>
<p>Xavier<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Le 21/11/2016 à 13:25, Dr. K. C. Bhamu
a écrit :<br>
</div>
<blockquote
cite="mid:CAJYci+Sx_5p7kJSiL+1GWGA0tcZE+RW9fPrsNUtbQ5MhHUBGZQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>
<div>
<div>
<div>
<div>
<div>
<div>Dear Prof. Peter,<br>
<br>
</div>
This is in the queue on my previous query regarding
CH3NH3PbI3:<br>
<br>
</div>
I used two strategy and got different results:<br>
<br>
<br>
<font color="#000000">The experimental band gap is
1.67 eV for orthorhombic lead halide perovskite.<br>
</font><br>
<div>I ran two cases (with PBE);<br>
</div>
1. with 1000 k, div: ( 11 7 11) and "min -j 'run_lapw
-p -I -NI -i 120 <b><span style="color:rgb(0,0,255)">-ec
</span></b>0.00001 <b><span
style="color:rgb(0,0,255)">-ec </span></b> 0.0001
-fc 1<br clear="all">
<div dir="ltr"><font color="#000000">2. with 400 k,
div: ( 8 5 8) and min -j 'run_lapw -p -I
-NI -i 120 </font><font color="#000000"><b><span
style="color:rgb(0,0,255)">-ec </span></b>
0.0001 </font><font color="#000000"><b><span
style="color:rgb(0,0,255)">-ec </span></b>
0.0005 -fc 2'<br>
</font><br>
<font color="#000099"><span style="color:rgb(0,0,0)">-ec
switch is used twice instead of ec and cc (it is
by mistake)</span><br>
</font></div>
<div><font color="#000000"><br>
</font></div>
<div dir="ltr"><font color="#000000"><br>
</font>
<div dir="ltr"><font color="#000000">Results:</font></div>
<div dir="ltr"><font color="#000000"><br>
</font></div>
<div dir="ltr"><font color="#000000">1. GAP 1.687
eV, FER: 0.1030934295, ENE: -339059.11079128</font></div>
<div dir="ltr"><font color="#000000">2. GAP 1.777
eV, FER: 0.1004906191, ENE: -339059.12432403</font></div>
<div dir="ltr"><font color="#000000"><br>
</font></div>
<div>In case 2 with less k-points and normal scf
criteria (<span style="color:rgb(0,0,0)"><span
style="color:rgb(0,0,0)"><span
style="color:rgb(0,0,255)">-ec</span></span>
0.0001 <span style="color:rgb(0,0,255)">-ec</span>
0.0005 -fc 2), I got minimized ENE than case 1.
But the band gap is good from case 1.</span><br>
</div>
</div>
<br>
</div>
You see that band gap is in the error of /+- 0.01eV.<br>
<br>
</div>
<br>
</div>
I did nothing special, just reduced rmt (5% and further rmt
of "I" was reduced by 0.02).<br>
Please correct me what is wrong here. Because usually PBE
always underestimates the experimental band gap.<br>
<br>
</div>
Kind regards<br>
</div>
Bhamu</div>
<br>
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