[Pw_forum] test case with structure relaxation

[Wei Lai]

Dear QE users,

I am new to quantum espresso and was playing with a simple test case of
variable cell relaxation. I created a virtual tetragonal MgO cell with c/a
ratio as 1.4. I was expecting this ratio will become close to 1.4142
(cubic) after relaxation. However, I am getting values that are off this
theoretical value. I tried to play with different threshold parameters but
didn't see significant improvement. Did I miss an important parameter
somewhere? I am pasting the input file. Thanks.

Regards, Wei
Wei Lai
Michigan State University
East Lansing, MI 48824

 &control
    calculation = 'vc-relax',
    restart_mode='from_scratch',
    prefix='MgO',
    tstress = .true.
    tprnfor = .true.
    pseudo_dir = '/home/espresso-4.3.2/pseudo/',
    outdir='/home/wl2/tmp/'
nstep=100
 /
 &system
    ibrav=  7, celldm(1) = 5.6298, celldm(3) = 1.4, nat=  2, ntyp= 2,
    ecutwfc =30, input_dft='pw91'
 /
 &electrons
    mixing_mode = 'plain'
    mixing_beta = 0.7
    conv_thr =  1.0d-8
 /
 &ions
   ion_dynamics = 'bfgs'
   trust_radius_ini=1.0d-1
   trust_radius_max=1.0d-3
 /
 &cell
 cell_dynamics='bfgs'
 /
 ATOMIC_SPECIES
 Mg  24.305 Mg.pw91-np-van.UPF
 O   15.999 O.pw91-van_ak.UPF
ATOMIC_POSITIONS
 Mg 0.00 0.00 0.00
 O 0.00 0.00 0.70
K_POINTS (automatic)
3 3 2 0 0 0
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