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

[Fabio Costa]

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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