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

[Marcin Dulak]

Hi,

4.46*(1-0.9) is too large cell compression, but even reducing it does 
not help.
You can reproduce the problem with:
mpiexec gpaw-python `which ase` gpaw bulk -x 'rocksalt' VO -M 2.0,1.0 
--k-point-density=8.0 -p xc=PBE,width=0.01,h=0.16 -a 4.46 -F 5,0.5
python `which ase` gpaw bulk -x 'rocksalt' VO -s -a 4.46 -F 5,0.5
I see that vasp also behaves like that, seems standard DFT is unable to 
deal with that.

Marcin

On 05/22/12 12:03, Zhenhua Zeng wrote:
> Hi Jun,
>
> Consider the big scaling value in lattice constant (0.9), there may be
> some problem in you script. V is not a element that so U demanding.
> Basically, U is not necessary to get the approximate lattice constant of
> 3d transitional metal oxides. Why not post you script and keep whether
> your parameters are convergent or not?
>
> Zhenhua
>
> On 2012/5/22 11:31, 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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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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