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

[Matteo Cococcioni]

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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