[Wien] volume optimization

[Rocquefelte]

Dear Colleague,

To simplify the discussion I propose to take the example of NiO.
When you are doing LDA+U or GGA+U, the Hubbard term leads to a 
correction of the chosen energy levels (here 3d(Ni) states). As a 
consequence the interaction (orbital overlap) between 3d(Ni) and 2p(O) 
states is modified. The related chemical bond is thus dependent of the 
Hubbard term you are using. As a consequence, by increasing the U value, 
you increase the ionicity of the Ni-O bond and thus modify the related 
bond length.

To conclude, when I use LDA+U or GGA+U, I always consider a set of 
properties. I choose the Ueff value to reproduce one property and then I 
use this Ueff value to predict the others. For sure it can be 
problematic in some cases but it works quite nicely in general.

Best Regards

Xavier


Le 3/1/2013 7:47 PM, Zsolt Rak a écrit :
> Dear wien2k users,
>
> I am calculating the properties of several f-electron compounds. I 
> would like to ask the users' opinion about the volume optimization in 
> an f- or d-electron system: which way is better (or physically 
> justified), with LDA/GGA or with LDA+U/GGA+U? In my opinion, the 
> LDA+U/GGA+U techniques were developed to correct band energies of 
> localized states, so there is no fundamental physical reason to use 
> LDA+U/GGA+U methods for volume optimization. However, we observe a 
> change in the lattice parameters when we go from LDA/GGA to 
> LDA+U/GGA+U. Also, from a brief search of the literature we found 
> that, in many cases, people tune the Hubbard-U parameter to reproduce 
> the experimental lattice constants.
> I would appreciate further thoughts and insights into this issue.
>
> Thank you,
> Zs
>
>
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