[Pw_forum] Problem with newly implemented LDA -1/2 in atomic code (ld1)

[Rajan Pandey]

Hi Leonardo,

Thanks a lot for the detailed description of LDA-1/2 in the atomic code. I
appreciate it.
Indeed, with CUT value of 3.67, I got the correct band-gap of Si. I went
through one iteration of the paper,
and I understand that the parameter CUT has to chosen such that it optimizes
the excitation energy. However,
It seems that there is no general rule for specifying the fraction of
electronic charge to be removed from the valence electronic configuration.
For group IV element it is 0.25 (C is), on the other hand for II-VI or III-V
compound it is 0.5 for anion p or cation d. What about the transition metal
with incomplete d orbital (for example Ti, Zr, V, Cr, Mn)? The only
transition metal which was discussed in the paper
is Zn, which has completely filled d orbital. Also, what about the
fractional charge to be removed from the valence of f-block elements?

Thank you, and best regards,

Rajan

On Thu, Mar 31, 2011 at 1:38 AM, Leonardo Matheus <leommj at usp.br> wrote:

> Hi,
>
> I'm Leonardo M M Jorge, the one who put the LDA-1/2 method on QE. And
> yes, it is working!
>
> You indeed have to create a new pseudopotential with ld1.x and the
> flag "iswitch=4" (among other options, shown in the example file
> atomic_doc/pseudo-LDA-0.5/si.lda.0.5.in ), and then use it on a
> standard LDA calculation.
>
> You obtained results very similar to standard LDA because one of these
> other options is not well set. When generating the pseudo, besides the
> electronic configuration used (of a half-ionized atom, hence the name
> -0.5 ), there is also another parameter needed: rcutv, which is the
> radius where you cut the perturbation. The value set on the example
> file (1.0) is too small; for Silicon these values should be somewhere
> around 3.6.
>
> To obtain the right value for this rcutv parameter you need to do
> several calculations, changing it by small steps, and then get the
> rcutv where the gap of the system is an extremum (maximum or minimum).
> The explanation why this works is written on the reference you just
> cited ( L. G. Ferreira, M. Marques and L. K. Teles, Phys. Rev. B 78
> 125116 (2008) ), but what's important is that it IS NOT a free
> parameter (one you keep changing until you get the result you want),
> as there is a procedure to obtain it.
>
> Also, as small variations of the rcutv parameters should give very
> small variations of the Electronic Gap (as it is an extremum for
> variations), we assume that you can obtain the right rcutv value for a
> small and simple system, and then use it on a bigger and more complex
> system (for a given atom; different atom kinds have different rcutv!).
>
> I hope I could explain the method for you, if you have any more
> questions feel free to write me.
>
> Leonardo Jorge
> PhD Student
> Universidade de São Paulo - Brazil
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