[Pw_forum] LDA+U calculation, difference in U values of one sort of atom.

[Nicola Marzari]

  On 10/13/10 2:03 PM, Maxim wrote:
> Dear pwscf users,
>
> I have obtained Hubbard U values for Ta atoms in Ta2O5 system. The
> values are 1.9 eV and 2.8 eV for two not equivalent atoms. I am confused
> about such relevant difference in the the values. As I understand
> Hubbard U characterizes the sort of atom and its coordination should not
> influence on the value.
> Could you help me to figure out what is wrong?
>
> Thank you in advance,
> Maxim


Dear Maxim,

I'd rather take the opposite view - each atom has its own
"U", that depends in principle on everything - even as you stretch
a bond the U will change, but it is always well-defined, and fully
ab-initio (following the Cococcioni/de Gironcoli PRB 2005).

Some delicate points to consider are the fact that you cannot
really compare energies for different values of U, in the functional,
and so you need, at the end, to choose an average U that is as much
as possible in the right ballpark.

Note that in most cases that works very well, but if you go through
significant changes in coordination/bond breaking, you might go
through electronic states where the value of U calculated by linear
response is very different from your ballpark.

We have written a few things on this with Heather Kulik (PRL and JCP).

                 nicola


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Prof Nicola Marzari   Department of Materials Science and Engineering
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