[Wien] Charged supercells

[Daniel Fernández Hevia]

Dear WIEN2k users and developers,

I have a couple of doubts about charged supercells.

1 For the case of charged isolated charged impurities, my question is if 
the next sequence of steps is correct:
1.1 Construct the supercell with all the neutral atoms and run (manually, 
not with w2web) instgen.
1.2 Edit the case.inst file and I add/remove electrons in the electronic 
shell structure of the impurity atom.
1.3 Then, I go on with the whole initialization process by typing 
init_lapw. Up to now, lstart always complains, issuing a message like
NUMBER OF ELECTRONS NE IZ               7.00000000000                       8
but, up to now, this has never been a problem: I just ignore this message 
and go through all the initialization process.
1.4 Check that the proper number of electrons appear in case.in2c. Also, I 
change the parameter bgch in case.inm (for example, if I removed 1 electron 
from an oxygen atom, then I type a -1 in case.inm). By the way, is there 
any stringent format for this parameter? I mean, I have exactly the 
following in the first line of the case.inm file:
BROYD -1.0  YES
With 1 space between “D” and the “-“ and two spaces between the “0” and the 
“Y” ... is that correct?
1.5 Finally, I run the scf.

According to the few discussions of these procedure that I have found in 
the mailing list, I would have no doubt that this is the correct procedure. 
However, I found a message from Prof. Blaha dated 7 March 2002 in which he 
answered a doubt “question on charged systems” by saying:
“ ... In order to make this properly in WIEN, you should "specify" the missing
charge in  case.inm    (Otherwise mixer renormalizes the interstitial charge).
In addition, I would never again run lstart/dstart (or init_lapw), but just
edit case.in2, remove the desired number of electrons and then  run_lapw.”
So now I am a bit confused and I should like to know if the above described 
process is generally correct or if there can be exceptions. Should I, or 
should I not, run lstart/dstart?

2 My second doubt deals with case of a charged vacancy or an impurity 
complex. I have not found information about this in the mailing list. The 
point is ... where do I add/remove electrons in the case.inm file in these 
cases? Say I study the N vacancy in AlN. With all the neutral atoms I find 
the expected additional eigenvalues: a a1 (s-like) state just above the 
valence band (occupied by two electrons) and three degenerate t2 (p-like) 
states, which are resonances in the conduction band, and are occupied by 
just one electron. Now, I should like to remove this last electron, 
creating a VN1+, i.e., the vacancy in a 1+ charge state. How do I do it? 
Should I remove one electron of, say, one of the four neighboring Al atoms? 
The same question raises for impurity clusters: If I work with, say, a MgO 
cluster in AlN, and I find that occupied states appear in the band gap, how 
do I give the cluster a 1+ charge state? Is there a rule for deciding from 
which one of the atoms in the cluster should I remove 1 electron?

Of course, any struct or whatsoever file in which you can be interested is 
at your disposal: just ask me.

I thank you in advance for any comments, suggestions or ideas about these 
topics: this mailing list is really an invaluable source of knowledge!!!

Best regards

Daniel

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Daniel Fernández Hevia
School of Physics
The University of Sydney
Sydney 2006, Australia
Phone: +61 2 9036 5301
email: <mailto:danielhf at physics.usyd.edu.au>danielhf at physics.usyd.edu.au
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