[Pw_forum] DFT+U: different oxidation states on the same element (charge ordering)

Mauro Sgroi maurofrancesco.sgroi at gmail.com
Tue Jun 10 12:40:28 CEST 2014


Dear Mehmet,
thanks a lot for the reply. I tried using PAW but nothing changed. So it
should be something related to other aspects of my input file.
Best regards,
Mauro.



2014-06-09 12:22 GMT+02:00 Mehmet Topsakal <mtopsaka at umn.edu>:

> I think the problem might be related with iron ultrasoft pseudopotential.
>
> As in page 40 of this pdf :
>
> http://www.vasp.at/vasp-workshop/slides/pseudopp2.pdf
>
> Low-spin case (NM) is okay. But for high-spin iron, energies are too
> different.
>
> VASP use PAW potential. So you should switch to PAW with QE.
>
> Hope this helps.
> Dear Matteo,
> thanks a lot for the reply.
> I tried also using occupations="smearing" but the situation is the same. I
> get a difference of 0.3 electrons between the iron atom Fe1 and the other
> three Fe2.
> Is there anything else that I can try? Could this behaviour due to the
> differences between VASP (that you used in the refererence article) and
> Quantum Espresso?
> Best regards,
> Mauro.
>
>
>
> 2014-06-06 16:55 GMT+02:00 Matteo Cococcioni <matteo at umn.edu>:
>
>>
>> Dear Mauro,
>>
>> one thing I notice is that you are using occupation fixed. is there a
>> reason for that? this, I think, might actually cause the result you are not
>> happy about.
>>
>> Matteo
>>
>>
>> On Fri, Jun 6, 2014 at 4:39 PM, Mauro Sgroi <
>> maurofrancesco.sgroi at gmail.com> wrote:
>>
>>> Dear all,
>>> I'm trying to study the phase separation in LixFePO4 using DFT+U.
>>> The reference article that I'm using is PRB 69, 201101 (R) 2004, "Phase
>>> separation in LixFePO4 induced by correlation effects, Zhou et al.
>>>
>>> The authors compare, using VASP, standard DFT with DFT+U and obtain the
>>> correct description of the material with the latter approach. In particular
>>> they found that is possible to describe the formation of two type of ion,
>>> Fe2+ and Fe3+, according to to the different position in the unit cell in
>>> the partially lithiated structures. With GGA all Fe ions have the same
>>> occupancies, regardless the position in the cell and the fact that they are
>>> more or less near a lithium ion.
>>> To obtain this result with DFT+U they had to force the breaking of the
>>> symmetry of the structure and, I imagine, they set the initial occupations
>>> in some way to force the desired electronic configuration.
>>>
>>> I'm trying to do the same, differentiating the Fe atoms, breaking the
>>> symmetry and using starting_ns_eigenvalue to force the initial occupations.
>>> My problem is that at the end of the calculation I got always the same
>>> occupations on the 4 Fe atoms in the structure (more or less, the
>>> difference could be 0.3 electrons).
>>>
>>> I'm using lda_plus_u_kind=0, but also using the other approach was not
>>> beneficial.
>>>
>>> My input file is below.
>>> Have you any suggestion?
>>>
>>> Thanks a lot in advance and best regards,
>>> Mauro Sgroi.
>>>
>>> &control
>>>     calculation='scf',
>>>     pseudo_dir = "/usr2/sgroi/DATABASE/ESPRESSO",
>>>     prefix='lifepo4_scfU',
>>>     outdir='/home/dati/espresso/temp',
>>>  /
>>>  &system
>>>     ibrav=8, celldm(1)=19.748515, celldm(2)=0.586193,
>>> celldm(3)=0.441599, nat=27, ntyp=5, nosym=.true.
>>>     ecutwfc=40, ecutrho=400, nbnd=130, occupations='fixed',  nspin=2,
>>> tot_magnetization=17,
>>>     lda_plus_u=.true., Hubbard_U(2)=4., Hubbard_U(3)=4.475,
>>>     starting_ns_eigenvalue(5,2,2)=0,
>>>     starting_ns_eigenvalue(4,2,2)=0,
>>>     starting_ns_eigenvalue(3,2,2)=0,
>>>     starting_ns_eigenvalue(2,2,2)=0,
>>>     starting_ns_eigenvalue(1,2,2)=0,
>>>     starting_ns_eigenvalue(5,2,3)=1,
>>>     starting_ns_eigenvalue(4,2,3)=0,
>>>     starting_ns_eigenvalue(3,2,3)=0,
>>>     starting_ns_eigenvalue(2,2,3)=0,
>>>     starting_ns_eigenvalue(1,2,3)=0
>>>  /
>>>  &electrons
>>>      conv_thr=1.0d-8, electron_maxstep=100, mixing_beta=0.5
>>>  /
>>>
>>> ATOMIC_SPECIES
>>>  Li 6.941   Li.pbe-s-van_ak.UPF
>>>  Fe1 55.847  Fe.pbe-sp-van_ak.UPF
>>>  Fe2 55.847  Fe.pbe-sp-van_ak.UPF
>>>  P  15.9994 P.pbe-van_ak.UPF
>>>  O  30.9737 O.pbe-van_ak.UPF
>>>
>>> ATOMIC_POSITIONS {crystal}
>>> Fe1      0.281537138   0.250000000   0.981851206    1   0   1
>>> P        0.103893879   0.250000000   0.415586970    1   0   1
>>> O        0.119764467   0.250000000   0.744164738    1   0   1
>>> O        0.447356144   0.250000000   0.201257756    1   0   1
>>> O        0.187375840   0.060350405   0.283173864
>>> Li       0.500000000   0.000000000   0.500000000    0   0   0
>>> Fe2      0.211199573   0.750000000   0.474632077    1   0   1
>>> P        0.405092969   0.750000000   0.922566801    1   0   1
>>> O        0.401164833   0.750000000   0.251505445    1   0   1
>>> O        0.041727760   0.750000000   0.686207216    1   0   1
>>> O        0.336134397   0.956851552   0.790300073
>>> Li       0.000000000   0.500000000   0.000000000    0   0   0
>>> Fe2      0.719627852   0.750000000   0.020422522    1   0   1
>>> P        0.902369089   0.750000000   0.575773635    1   0   1
>>> O        0.896492242   0.750000000   0.246982466    1   0   1
>>> O        0.540334658   0.750000000   0.790913565    1   0   1
>>> O        0.840097513   0.539754373   0.720678002
>>> Li       0.500000000   0.500000000   0.500000000    0   0   0
>>> Fe2      0.786894270   0.250000000   0.525238732    1   0   1
>>> P        0.588956858   0.250000000   0.078832190    1   0   1
>>> O        0.587126133   0.250000000   0.749458661    1   0   1
>>> O        0.966454798   0.250000000   0.315230191    1   0   1
>>> O        0.654364669   0.453679217   0.216461848
>>> O        0.822813149   0.944845411   0.700568536
>>> O        0.653714964   0.047069624   0.216100948
>>> O        0.172021112   0.453011228   0.263457462
>>> O        0.335493673   0.543326856   0.788919121
>>>
>>> K_POINTS {automatic}
>>> 2 4 8 1 1 1
>>>
>>>
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