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

[Leonardo Matheus]

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