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

[jun yan]

Hi Marcin, 

The 0.9 scaling factor means 4.46 * 0.9 ….
Thanks for the command line script ! 
I agree its a DFT problem. It will be great if anyone can give any suggestions on how they deal with these problems. 

All the best, 
Jun

On May 22, 2012, at 7:44 AM, Marcin Dulak wrote:

> 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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> 
> -- 
> ***********************************
> 
> 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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