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

[Marcin Dulak]

Hi,

if you fix the moment without +U, that gives minima:
mpiexec gpaw-python `which ase` gpaw bulk -x 'rocksalt' VO -M 2.0,1.0 
--k-point-density=6.0 -p xc=PBE,width=0.01,h=0.16,fixmom=True -a 4.46 -F 
5,1.0
but fixing the moment to 2 instead of 3 gives lower energy, and a 
smaller lattice constant:
mpiexec gpaw-python `which ase` gpaw bulk -x 'rocksalt' VO -M 1.0,1.0 
--k-point-density=6.0 -p xc=PBE,width=0.01,h=0.16,fixmom=True -a 4.3 -F 
5,1.0
and fixing magnetic moment to 1 gives a lower energy than moment of 3, 
but higher than moment of 2, and lattice constant even smaller than that 
of moment of 2:
mpiexec gpaw-python `which ase` gpaw bulk -x 'rocksalt' VO -M 0.5,0.5 
--k-point-density=6.0 -p xc=PBE,width=0.01,h=0.16,fixmom=True -a 4.25 -F 
5,1.0

Marcin

On 05/22/12 17:14, jun yan wrote:
> 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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-- 
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Marcin Dulak
Technical University of Denmark
Department of Physics
Building 307, Room 229
DK-2800 Kongens Lyngby
Denmark
Tel.: (+45) 4525 3157
Fax.: (+45) 4593 2399
email: Marcin.Dulak at fysik.dtu.dk

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