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

[José Carlos Conesa]

Hi,

What I could verify is that, in a simple test on rutile TiO2, I obtain 
the same total energy (to within 10**(-7) Ry) with any of these lines in 
the input file:

input_dft='sla+pw+tb09+tb09'

input_dft='pw+pbc+tb09+tb09'

input_dft="noc+nogc+tb09+tb09"

The first case is the one specified in the Modules/funct.f90 file as 
equivalent to input_dft='tb09', the second one corresponds to having gga 
correlation, the last one, to using no correlation at all. One can 
understand this since the said file contains these lines:

     ! special case : TB09 meta-GGA Exc
     else IF ('TB09'.EQ. TRIM(dftout) ) THEN
        dft_defined = set_dft_values(0,0,0,0,0,3)

I think that since the original mBJ functional was defined as including 
LDA correlation, this should be changed. Preferably, by allowing the 
user to specify the type of correlation (s)he desires. I wonder if just 
suppressing in the said file the three lines mentioned above would be 
enough.

Regards,

JC Conesa

El 05/05/2018 a las 10:44, Paolo Giannozzi escribió:
> I don't think it is possible right now to use any meta-GGA with 
> exchange/correlations/gradient corrections that differ from those 
> assumed in the original definition of the meta-GGA functional. 
> Meta-GGA follow a different path from all other XC functionals. I hope 
> to manage sooner or later to clean up the meta-GGA mess, but cannot 
> guarantee anything.
>
> Paolo
>
> On Fri, May 4, 2018 at 6:33 PM, José C. Conesa <jcconesa at icp.csic.es 
> <mailto:jcconesa at icp.csic.es>> wrote:
>
>     Hi,
>
>     Does this mean that it is not possible to combine tb09 with any
>     correlation contribution?
>
>     José Carlos
>
>
>     El 04/05/2018 a las 17:41, Paolo Giannozzi escribió:
>>     I think that with meta-GGA what is written in the first two
>>     fields is just ignored
>>
>>     Paolo
>>
>>     On Fri, May 4, 2018 at 5:39 PM, José C. Conesa
>>     <jcconesa at icp.csic.es <mailto:jcconesa at icp.csic.es>> wrote:
>>
>>         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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>>
>>
>>     -- 
>>     Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
>>     Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>>     <https://maps.google.com/?q=Udine,+via+delle+Scienze+208,+33100+Udine,+Italy&entry=gmail&source=g>
>>     Phone +39-0432-558216, fax +39-0432-558222
>>
>>
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>
>     -- 
>     José C. Conesa
>     Instituto de Catálisis y Petroleoquímica, CSIC
>     Marie Curie 2, Cantoblanco
>     28049 Madrid, Spain
>     Tel. (+34)915854766
>
>
>
>
> -- 
> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>

-- 
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Madrid, Spain
www.icp.csic.es
Tel. (+34)915854766

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