[Pw_forum] equilibrium lattice constant of magnetite

[Abolore Musari]

Dear QE users
Sir, I am try to find the lattice constant of magnetite (Fe3O4) which is a
spinel ferrimagnetic material. The expt lattice constant was 15.87 and I
have been trying to find the equilibrium lattice constant (celldm(1))
between 15.80 - 16.00 and all i get is a straight line graph instead of the
usual exponential curve graph. I have tried all l could most esp with
starting_magnetization but the results are the same. My input is pasted
below pls help me to get the appropriate graph.
The kind of the graph I got is attached.
Thanks

&control
    calculation = 'scf',
    restart_mode ='from_scratch',
    prefix = 'Fe3O4',
    pseudo_dir = '/home/abolore/Programs/Pseudos/',
    outdir = './tmp/'
 /
 &system
    ibrav = 2,
    celldm(1)= $15.80-16.00,
    nat = 3,
    ntyp = 3,
    ecutwfc = 40,
    ecutrho = 400,
    starting_magnetization(1) = 0.7,
    starting_magnetization(2) = -0.5,
    starting_magnetization(3) = 0.0,
    nspin = 2,
    occupations='smearing',
    smearing='gaussian',
    degauss = 0.05,
 /
 &electrons
   diagonalization = 'david',
   mixing_mode = 'plain',
   mixing_beta = 0.7
 /
ATOMIC_SPECIES
    Fe1     55.845   Fe.pz-nd-rrkjus.UPF
    Fe2     55.845      Fe.pz-nd-rrkjus.UPF
     O      16.000      O.pz-rrkjus.UPF

ATOMIC_POSITIONS
Fe1 0.125  0.125  0.125
Fe2 0.500  0.500  0.500
O    0.2548 0.2548 0.2548

K_POINTS (automatic)
  8 8 8 1 1 1
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