[Pw_forum] U calculation in GGA+U

[Matteo Cococcioni]

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