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<div class="moz-cite-prefix">Dear Colleague,<br>
<br>
To simplify the discussion I propose to take the example of NiO. <br>
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.<br>
<br>
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.
<br>
<br>
Best Regards<br>
<br>
Xavier<br>
<br>
<br>
Le 3/1/2013 7:47 PM, Zsolt Rak a écrit :<br>
</div>
<blockquote
cite="mid:CACQkguoNr39t0_fVz4HrHkuxLQoTg_4OxcYfij=_BzvzAHLT5w@mail.gmail.com"
type="cite">Dear wien2k users, <br>
<br>
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. <br>
I would appreciate further thoughts and insights into this issue.<br>
<br>
Thank you, <br>
Zs <br>
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