[QE-users] on the use of tb09 functional from libxc

[José C. Conesa]

Dear QE developers,

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

input_dft="sla+pw+tb09+tb09"

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

input_dft="pw+pbc+tb09+tb09"

(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.

Can you please comment on this?

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.

All the best,

-- 
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Cantoblanco
28049 Madrid, Spain
Tel. (+34)915854766



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