[Pw_forum] U calculation in GGA+U

[Peng Chen]

Dear Dr. Cococcioni,

Thanks a lot for the reply. It is very clear. I appreciate it.

On Sun, Dec 2, 2012 at 12:25 PM, Matteo Cococcioni <matteo at umn.edu> wrote:

> Dear Peng,
>
> I'm not entirely sure about what you mean by "site symmetry".
> Anyway, if two atomic positions are not equivalent by (point group)
> symmetry, then the U will likely be different and should be recomputed
> for each of them. This is the case of Fayalite.
> Some times you can avoid recomputing the U if the only difference
> between the atomic sites is the sign of magnetization. This is the
> case of FeO or other transition-metal oxides with the same
> stoiciometry. In these systems you have two TM atoms in the unit cell
> one with spin up the other with spin down (but the same absolute
> magnetization: in fact the material is AF).
> If you are not sure about what to do, a good way to decide whether you
> need to recompute U or not is to look at the atomic occupations
> printed by the code (which are, in fact symmetrized). If two atoms
> have different occupations (the difference has to be larger than
> possible numerical noise, usually affecting occupations at the 4th
> decimal point) and their occupations cannot be obtained one from the
> other just interchanging majority with minority spins, then you need
> to recompute U.
>
> Regards,
>
> Matteo
>
> On Sun, Dec 2, 2012 at 11:02 AM, Peng Chen <pchen at ion.chem.utk.edu> wrote:
> > Dear All,
> >
> > In the calculations of U, I remember that I got some advice from the
> forum
> > that  the equivalence of the ions is determined by  space group symmetry
> not
> > site symmetry.   E.g  AB2O3, B ions has 2 different site symmetries (B(1)
> > B(2)).  We should only run the calculation on perturbing B ions. But from
> > Dr. Matteo Cococcioni's paper (PRB 71, 035105 (2005)),  both Fe1 and Fe2
> of
> > fayalite are perturbed and have different U values. I think Fe1 and Fe2
> are
> > different in site symmetry. So I am just curious which  is the best way
> to
> > do the same kind of calculations.
> >
> > --
> >   Best Regards.
> >         Peng
> >
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>
>
>
> --
> Matteo Cococcioni
> Department of Chemical Engineering and Materials Science,
> University of Minnesota
> 421 Washington Av. SE
> Minneapolis, MN 55455
> Tel. +1 612 624 9056    Fax +1 612 626 7246
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>



-- 
  Best Regards.
        Peng
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