[Pw_forum] U

[Matteo Cococcioni]

Dear Ali

I just want to add something what Nicola, Giuseppe and Gabriele wrote.
it is a good idea to reoptimize the structure after computing U. If you 
only need to optimize the lattice parameter
(the atomic positions are fixed) this is easy: you just need to compute 
U for a series of different
lattice parameter and then fit E vs V on a Murnaghan eq of state for 
example.
If the relaxation of internal degrees of freedom is relevant there is a 
further level of consistency between U and
the structure. In principle for each volume (or each pressure) you need 
to iterate the calculation of U with the internal relaxation.
Hopefully U won't change much and an (average) U will work for all 
lattice parameters (as Nicola suggests) in your case. However
we have recently found that a structurally consistent U has a finite 
effect, if not on energies, on their derivative wrt to V, i.e. pressure.
see the paper PRB 79, 125124 (2009) for details.

regards,

Matteo



Nicola Marzari wrote:
> Giuseppe Mattioli wrote:
>   
>> I suggest you to reoptimize the lattice parameters. You could find quite 
>> different results with respect to the LDA or GGA results.
>>     
>
>
> Also, as a matter of caution - do use a single U value in your
> equation of state - ideally what you'd get from Cococcioni procedure
> close to the minimum of the DFT+U equation of state.
>
> 			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
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>   


-- 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%