[QE-users] On the use of the modified Becke-Johnson (TB09) functional

[Fabrizio Ferrari]

Dear Fabio,
there has been quite recently a 'more consistent' incorporation of libxc
functionals into the QE code (look at 'develop' branch in GitLab).
In order to use them, as you already did, you have to put their names into
the input_dft string, but it is necessary to write their complete name, so,
for example, in your case it is MGGA_X_TB09, not just TB09 ( a complete
list of the names is shown at:
https://www.tddft.org/programs/libxc/functionals/ ).
However, that functional belongs to a subset of functionals that need
additional information with respect to most of the other cases in order to
be properly used so, at the moment, there are no guarantees on their
correct operation.

Best regards,
Fabrizio

On Mon, Oct 14, 2019 at 3:11 PM Fabio Costa <fabiocosta4 at hotmail.com> wrote:

> Dear all
>
> I'm having some difficulties to work with the TB09 functional. After some
> reading, I found in  some old posts that this can be done by compiling QE
> with Libxc, and by adding the line 'input_dft='tb09'' in the pw.x input.
>
> When I try to use this functional, the scf calculation always ends like
> this:
>
>      total energy              =              NaN Ry     Harris-Foulkes
> estimate   =     -11.24358313 Ry     estimated scf accuracy    <
> 0.01858366 Ry     iteration # 18     ecut=   120.00 Ry     beta= 0.10
>  Davidson diagonalization with
> overlap %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>    Error in routine cdiaghg (7):     eigenvectors failed to
> converge %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>
> What draws my attention is that always in the last cycle before the error
> the total energy is printed as NaN. The parameters to the calculation seem
> to be fine, as it ends smoothly without the input_dft line.
>
> By searching in the QE user guide, there is a reference to the work of E.
> Germaneau, which lead me to his paper (DOI: 10.1016/j.cpc.2013.02.020
> <http://dx.doi.org/10.1016/j.cpc.2013.02.020>), where the results of his
> implementation are presented. Even though, it is unclear to me how to use
> the functional correctly.
>
> Thanks for any assistance
> Fábio Costa
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