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

[jun yan]

Hi Ole, 

 Thanks for the reply. Its very useful.  Just to verify if I didn't misunderstand, so It means that if I would fix point of magnetic moment 3, then U will change with different lattice parameters that I use for equation of state ?  Do you know how do the lattice parameters compared with experiments when a certain U can give the correct magnetic moment ? (this question can also be phased in this way : can U gives the right lattice parameters and magnetic moment at the same time ?)  For more complex systems, how do one know what magnetic moment they should expect ?  Thanks for any thoughts, 

All the best, 
Jun

On May 22, 2012, at 2:31 AM, Ole Jess Stausholm-Møller wrote:

> 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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> 
> 
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