[Wien] Bad formation energies for the charged vacancies

[Yurko Natanzon]

Dear Wien2k users and developers,
I'd like to refresh the discussion about the total energies of the
charged cells which took place three years ago:
http://zeus.theochem.tuwien.ac.at/pipermail/wien/2007-January/008711.html

I'm trying to calculate the formation energy of the Hydrogen vacancy
in +/-1 charge states and find that the results are bad (much differ
from the literature) although the formation energy of the neutral
hydrogen vacancy is good. So my question arises if we can trust the
values of the total energies for the charged cells in the recent
version of Wien2k?

To investigate this issue further I have performed the following
tests: I've done the calculations of the total energy of Mg, MgH2 and
GaN for three cases: neutral cell, cell with one electron removed (+1
charge) and a cell with an electron added (-1 charge). The results
were compared with the same calculation with another plane-wave code
and are the following:
-----------------------------------------
hcp Mg:
Wien2k:
E(+1)-E(0) = 0.245 Ry
E(-1)-E(0) = -0.199 Ry

Plane-Wave code:
E(+1)-E(0) = -0.226 Ry
E(-1)-E(0) = 0.281 Ry

bcc MgH2
Wien2k:
E(+1)-E(0) = 0.277 Ry
E(-1)-E(0) = 0.085 Ry

Plane-Wave code:
E(+1)-E(0) = 0.024 Ry
E(-1)-E(0) = 0.326 Ry

fcc GaN
Wien2k:
E(+1)-E(0) = 1.12 Ry
E(-1)-E(0) = -0.717 Ry

Plane-Wave code:
E(+1)-E(0) = -0.151 Ry
E(-1)-E(0) = 0.443 Ry
-------------------------------------------------

In wien2k the charged cell was created by changing the number of
electrons in case.in2 and adding the corresponding background charge
in case.inm. One can observe, that the energies have the same order of
magnitude, but the sequence of energies E(+1), E(0) and E(1) is
inverse. It seems, that the system with +1 charge (electron removed)
behaves like the system with -1 charge in the Plane-Wave code. Of
course, the results of tests are not physical, because no supercell
was used and no geometric relaxation was performed (however, it is not
needed for Mg), but if one tries to do all the supercell and
relaxation stuff and tries to calculate the defect formation energy,
the result will be the same.

I'd be grateful if you comment on this and suggest any corrections
which should be provided to the total energies for the charged cells.

with kind regards,
Yurko


-- 
Yurko (aka Yuriy, Iurii, Jurij etc) Natanzon
PhD student
Department for Structural Research (NZ31)
Henryk Niewodniczański Institute of Nuclear Physics
Polish Academy of Sciences
ul. Radzikowskiego 152,
31-342 Krakow, Poland
E-mail: Yurii.Natanzon at ifj.edu.pl, yurko.natanzon at gmail.com