[gpaw-users] Lattice parameter with GGA or GGA+U ?

[Ole Jess Stausholm-Møller]

Typically the +U gives larger lattice parameters, and these typically  
scale with the value of U applied, so as you increase U you also  
increase the lattice parameters. Naturally the larger value of U the  
larger magnetic moments you typically also get. So I guess that you  
could use the +U, but you have to have some reason to use a particular  
range of U values. Say your fix point is a magnetic moment of 3, then  
you should use the U value that gives you this magnetic moment and  
then check the sensitivity of your results the applied value of U.

/Jess

Quoting jun yan <junyan at stanford.edu>:

> Hi,
>
>      I am trying to get the lattice parameter for VO, which has a  
> very simple structure of atoms = bulk('VO', 'rocksalt', a=4.46)
> The following is what I got with PBE using spinpol=True.  The  
> OV_0.90000 means scaling lattice parameter a=4.46 with 0.9 .
>
> OV_0.90000.txt:Total Magnetic Moment: 1.956392
> OV_0.95000.txt:Total Magnetic Moment: 2.448952
> OV_1.00000.txt:Total Magnetic Moment: 2.600094
> OV_1.05000.txt:Total Magnetic Moment: 2.638621
>
> OV_0.90000.txt:Free Energy:    -16.24786
> OV_0.95000.txt:Free Energy:    -16.24359
> OV_1.00000.txt:Free Energy:    -16.20235
> OV_1.05000.txt:Free Energy:    -16.11876
>
> so as the lattice parameter becomes small, the total energy  
> decreases with decreasing magnetic moment. But this is wrong. The  
> expected lattice parameter will be around a=4.46, (scaling factor  
> 1.0) and with magnetic moment 3.0.
>
> My question is should I use GGA+U to get lattice parameters ?  If  
> so, should I use the same U for all the scaling factors ? I expect  
> that if the cell becomes smaller, the d-electrons interactions  
> change and thus requires a different U…  and even if I use the same  
> U, would different U gives different lattice parameter ?
>
>  I appreciate if someone with more experiences would like to shine  
> some light.
>
> All the best,
> Jun
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