[Pw_forum] Range of Hubbard U parameter for Fe

[Hooman Yaghoobnejad Asl]

Greetings QE developers and users,
I know this question is vague, but does anyone expect reasonable U values
for iron? following the linear response method of Cococcioni and Gironcoli,
I got U values for Fe in Na3Fe(II)PO4CO3 and Na2Fe(III)PO4CO3 (single Fe
site in the asymmetric unit) to  be 6.39  and 7.82 eV, which seems much
higher than other Fe containing compounds in phosphates (vaguely speaking).
This leads to positive deviations from experimental observations (e.g. XRD
cell volume and redox potential). can anyone please comment on possible
causes? the input file for unperturbed SCF calculations is included
(diago_thr_init = last value in zero alpha along with charge-density.dat
used for subsequent non zero perturbations).
Q.E. version: 5.2.1

&CONTROL
                       title = 'Na2FePO4CO3 U det' ,
                 calculation = 'scf' ,
                      outdir = '/etc./common/' ,
                      wfcdir = '/etc./common/' ,
                  pseudo_dir = '/etc./pseudo/' ,
                      prefix = 'Na2FePO4CO3U' ,
                   verbosity = 'high' ,
 /
 &SYSTEM
                       ibrav = 12,
                           A = 8.997 ,
                           B = 5.163 ,
                           C = 6.741 ,
                       cosAB = -0.0027925231 ,
                         nat = 10,
                        ntyp = 5,
                     ecutwfc = 30 ,
                     ecutrho = 270 ,
                  lda_plus_u = .true. ,
             lda_plus_u_kind = 0 ,
                Hubbard_U(1) = 1d-20,
            Hubbard_alpha(1) = 1d-20,
                 space_group = 11 ,
                     uniqueb = .false. ,
 /
 &ELECTRONS
                    conv_thr = 1d-9 ,
                 startingpot = 'atomic' ,
                 startingwfc = 'atomic' ,
             diagonalization = 'david'
 /
ATOMIC_SPECIES
   Fe   55.93300  Fe.pbe-sp-van_mit.UPF
    P   30.97400  P.pbe-van_ak.UPF
    C   12.01100  C.pbe-van_ak.UPF
    O   15.99900  O.pbe-van_ak.UPF
   Na   22.99000  Na.pbe-sp-van_ak.UPF
ATOMIC_POSITIONS crystal_sg
   Fe      0.362400000    0.279900000    0.250000000
    P      0.587500000    0.798900000    0.250000000
    O      0.569400000    0.096500000    0.250000000
   Na      0.740900000    0.251300000    0.002200000
    C      0.060500000    0.234500000    0.250000000
    O      0.123300000    0.461600000    0.250000000
    O      0.143000000    0.031700000    0.250000000
    O      0.919500000    0.215700000    0.250000000
    O      0.321000000    0.281700000    0.565500000
    O      0.431600000    0.674000000    0.250000000
K_POINTS automatic
  3 6 5   1 1 1

-- 

*Hooman Yaghoobnejad*

*Department of Chemistry*

*Missouri University of Science and Technology*

*Rolla, MO 65401*
*USA*
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