[Pw_forum] About LDA+U

[Nicola Marzari]

Paolo Giannozzi wrote:
> wangqj1 wrote:
> 
>> Does anyone know the Hubbard U parameter of ZnO and Mn ?
> 
> for what is worth: for Mn a "typical" U parameter is in
> the order of 4eV
> 
> Paolo

Dear Wangqj1,

as a side note to Paolo's comments:

1) the value of U is fairly sensitive to the oxidation state of
the pseudopotential - so the same element, generated with different
reference atomic configurations (e.g. neutral, +1, +2.5, etc...)
would give fairly different U, as calculated according to the
procedure of Cococcioni and de Gironcoli, PRB 2005. We have an
example in the appendix of Kulik and Marzari JCP 129 134314 (2008).

2) the U implemented in PWSCF is more like a U-J, so you might
want even more care in comparing this with other published results.

My own comment is that the numerical value of U shouldn't be
given too much importance. What is important is stating what is the
the analytic shape of the Hubbard term (e.g. PWSCf adopts the C&dG PRB
2005 rotationally-invariant functional), and agreeing that, for
a given structure, for a given electronic-structure configuration,
and for a given pseudopotential, the linear-response procedure
gives us a well-defined, first-principles U to use.

Last - extra-care should be paid to the fact that some of the 
pseudopotentials in the PWSCF database have non-normalized atomic 
wavefunctions - I've lost track of what is the latest status on
this (Matteo, Heather - can you comment ?) but I believe
norm-atomic would fix that automatically.

Best,

			nicola

---------------------------------------------------------------------
Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu