[Pw_forum] Hubbard U Calculation for Hematite

[Sai Kumar Ramadugu]

Dear QE Users,

I am trying to calculate the Hubbard U for bulk Fe2O3 using the linear
response method of Cococcioni et al.

The problem I am having as of now is that some of the responses dn0/da and
dn/da are not linear and do not intersect at 0.

I am following the approach suggested by Dr. Cococcioni in earlier posts
several people.
1. SCF calculation
2. Grep the ethr from previous SCF and run the alpha perturbations for Fe
and O.
3. Calculate U by inverse of chi0 and chi matrices.

I am using QE version 5.0.2 and below are the options of the input
criterion for the scf run:

 &control
   calculation = 'scf',
   restart_mode='from_scratch',
   prefix = 'Fe2O3',
   pseudo_dir='$ESPRESSO_PSEUDO',
   outdir='$temp',
   etot_conv_thr = 1.0D-6,
   forc_conv_thr = 1.0D-5,
&system
   ibrav = 0,
   nat = 30,
   ntyp = 4,
   ecutwfc = 35,
   ecutrho = 280,
   occupations = 'smearing',
   smearing = 'mp',
   degauss = 0.02,
   nspin = 2,
   starting_magnetization(1) =  0.5,
   starting_magnetization(2) = -0.5,
   starting_magnetization(3) =  0.5,
   lda_plus_u = .true.,
   U_projection_type = 'atomic',
   Hubbard_U(1) = 1.d-10,
   Hubbard_U(2) = 1.d-10,
   Hubbard_U(3) = 1.d-10,
   Hubbard_U(4) = 1.d-10,
/
 &electrons
   mixing_beta = 0.5,
   conv_thr = 1.0d-8,
/
CELL_PARAMETERS {bohr}
9.531782 0.000000 0.000000
-4.765891 8.254766 0.000000
0.000000 0.000000 26.259456
ATOMIC_SPECIES
  FE1  1.0  Fe.pbe-nd-rrkjus.UPF
  FE2  1.0  Fe.pbe-nd-rrkjus.UPF
  FE3  1.0  Fe.pbe-nd-rrkjus.UPF
  O    1.0  O.pbe-rrkjus.UPF
ATOMIC_POSITIONS {crystal}
  FE3  0.000000000  0.000000000  0.000000000
  FE2 -0.333333343 -0.666666687  0.119881678
.....
.....other atoms' coordinates
.....
K_POINTS {automatic}
 6 6 6 1 1 1


This run is followed by the alpha perturbations. I am using alpha values
ranging from +- 0.01 to +- 0.05 including 0.0 at intervals of 0.01.
Some responses are linear but most of them are not linear. I can attach the
figures if needed!

Any suggestions are greatly appreciated!

Thank you,
Sai


------

Sai Kumar Ramadugu,
Post-Doctoral Scholar,
University of Iowa.
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