[Wien] Charged supercells

[Stefaan Cottenier]

Nice explanation, wouldn't it be useful to put this in the FAQ ? This
question comes up from time to time...

Stefaan

----- Original Message ----- 
From: "Peter Blaha" <pblaha at zeus.theochem.tuwien.ac.at>
To: <wien at zeus.theochem.tuwien.ac.at>
Sent: Friday, May 14, 2004 8:59 AM
Subject: Re: [Wien] Charged supercells


> For charged supercells:
>
> 1) Setup your supercell, put impurities, vacancies,.. (all with neutral
atoms!)
>
> 2) Run a standard initialization (init_lapw) with NEUTRAL atoms.
> Don't fiddle around with case.inst (positively charged atoms may work (and
> one can neglect the warnings "NUMBER OF ELECTRONS NE IZ "), but most
> negative ions are unstable in DFT and will not work.
>
> 3) run_lapw  (again for neutral atoms !) . Save the results "save
case_neutral"
>
> 4) Edit case.in2: In the second line you see the number of electrons NE.
>    Add or remove an electron to get the desired charged state.
>
> 5) Edit case.inm: The first line must read:
>
> BROYD  1.0   YES        (when you added an electron at step 4     or
> BROYD -1.0   YES        (when you removed an electron at step 4
>
> The position of   "YES" is important!!! When reading characters in Fortran
> one must use a fixed format (or put characters into quotes).
> (In your mail the YES was misplaced)
>
> 6) run_lapw
>
> You see, I NEVER run   init_lapw again!!!  Usually  init_lapw should be
run
> only ONCE in a directory, because any changes which would require
rerunning
> init_lapw (symmetry changes,....) should be done in a fresh "case" (maybe
> except changing RMT, but for this purpose I'll provide another tool with
the
> next update).
>
> It is NOT necessary to start an scf cycle with a density which has the
> "correct" number of electrons for these charged cells.
>
> > I have a couple of doubts about charged supercells.
>
> > 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?
>
> I guess with my explanations above the answer to your point 2) is clear.
> You cannot (and need not) remove an electron from a particular atom.
> All you can specify is the total number of electrons in your supercell!!!
> During the scf cycle the system will decide itself if an electron
comes/goes
> from/to a particular site.
>
>                                       P.Blaha
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
> Email: blaha at theochem.tuwien.ac.at    WWW:
http://info.tuwien.ac.at/theochem/
> --------------------------------------------------------------------------
>
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