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

[Mauro Sgroi]

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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