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<p><font face="Helvetica, Arial, sans-serif">Dear QE developers,</font></p>
<p>I wish to use in Quantum Espresso the tb09 potential-only
meta-GGA functional (from Tran & Blaha, PRL 102, 2009, 226401)
which is available in libxc. I downloaded the libxc library and
compiled successfully qe-6.2.1 with it. Then I see in
Modules/funct.f90 that the instruction to use this functional is
equivalent to specifying <br>
</p>
<p>input_dft="sla+pw+tb09+tb09"</p>
<p>But as far as I know, the tb09 functional gives only the exchange
part, and gives it in full (not as a correction to another
expression for the exchange), while sla is a LDA exchange form
(and pw a LDA correlation form). Should one conclude that exchange
is being introduced twice? And, is a gradient correction to the
correlation term not included? From these considerations, I would
have thought that using something like <br>
</p>
<p>input_dft="pw+pbc+tb09+tb09"</p>
<p>(where I hope that the tb09 exchange functional is not included
twice), i.e. the correlation part of PBE plus the Tran & Blaha
exchange functional, would be the proper way to proceed.</p>
<p>Can you please comment on this?</p>
<p>I also would add that it would be good to be able to adjust the A
and B parameters of the tb09 functional. This has been proposed in
(Koller, Tran & Blaha, PRB 85, 2012, 155109) to obtain better
bandgaps in certain families of semiconductors.</p>
<p>All the best,<br>
</p>
<pre class="moz-signature" cols="72">--
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Cantoblanco
28049 Madrid, Spain
Tel. (+34)915854766</pre>
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