[Pw_forum] Possible bug in TB09 MetaGGA

[Pedro Miguel Castro Borlido]

Greetings,

Sorry for the long delay in responding...

I tried setting the values manually for a small set of c's (see data.dat in
attachment) with

call xc_f90_mgga_x_tb09_set_par(xc_func, cc)

and ran the silicon structure with the tpss pseudopotential in
the QE site.

Indeed, for c=1 I got the same value as the default calculation...
(And a rather large gap although that had already been mentioned in
another thread http://qe-forge.org/pipermail/pw_forum/2015-April/106571.html)


I also tried computing the c parameter as per eq. 3 (see attach. cc_tb09.f90).
For a (default) calculation with the same pseudo for Si I get cc = 1.0135.
 From the cc runs however, the 'optimal' value should be around 0.85 ~ 1.00,
so there is a certain discrepancy with the original article...


Just out of curiosity, is there a specific reason for the usage of TPSS in
correlation instead of the original PW92? The former seems to be rather
incompatible with non-tpss pseudos...


Quoting Paolo Giannozzi <p.giannozzi at gmail.com>:

> Hard to say. Since you already had a look at the problem, you are in the
> best position to find whether the code computes what it is expected to
> compute or not. Try to set c to 1, then to a different parameter, see what
> happens.
>
> Paolo
>
> On Wed, Feb 24, 2016 at 4:05 PM, Pedro Miguel Castro Borlido <
> pedro.borlido at uni-jena.de> wrote:
>
>>
>> Greetings,
>>
>>  I understand that the implementation of the TB09 meta gga is done
>> in Espresso
>> though Libxc, as described in the article by Germaneau and colleagues.
>>  However, while looking through the code it seems that at no point
>> the c parameter
>> (equation 3 of PRL 102, 226401 (2009)) is set with
>> xc_f90_mgga_x_tb09_set_par thus
>> leaving c equal to 1 by default (i.e. the original BJ is used).
>>
>> Could someone please check if this is actually a bug of if c is
>> actually set?
>>
>> Best regards,
>> Pedro Borlido
>>
>> Friedrich-Schiller Universität Jena
>> Institut für Festkörpertheorie und -optik
>> Max-Wien-Platz 1
>> 07743 Jena
>>
>>
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://pwscf.org/mailman/listinfo/pw_forum
>
>
>
>
> --
> 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

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