[Pw_forum] Relaxation doesn't converge

[Pablo Aguado]

Dear all,

I'm doing some tests on the structural optimization with pw. I'm
currently testing with the tetragonal structure of PbTiO3, using the
following input:

&control
    calculation  = 'vc-relax'
    restart_mode = 'restart'
    pseudo_dir   = '/****/espresso-4.1/pseudo/'
    outdir       = '/***/tmp'
    forc_conv_thr = 1d-3
    nstep = 50
 /
 &system
    ibrav=6
    celldm(1)=7.3699
    celldm(3)=1.04
    nat=5
    ntyp=3
    nbnd=28
    ecutwfc=30.0
    occupations = 'fixed'
 /
 &electrons
    conv_thr = 1d-12,
    mixing_beta=0.4,
 /
 &ions
    ion_dynamics = 'bfgs'
    bfgs_ndim = 5
 /
 &cell
    cell_dynamics = 'damp-pr'
    press_conv_thr = 1.0d0
    cell_dofree = 'z'
 /
ATOMIC_SPECIES
  Pb   207.2      Pb.pz-d-van.UPF
  Ti    47.867    Ti.pz-sp-van_ak.UPF
  O     15.9994   O.pz-van_ak.UPF
ATOMIC_POSITIONS crystal
  Pb    0.000    0.000    0.040   0 0 1
  Ti    0.500    0.500    0.520   0 0 1
  O     0.000    0.500    0.500   0 0 1
  O     0.500    0.500    0.000   0 0 1
  O     0.500    0.000    0.500   0 0 1
K_POINTS automatic
  6 6 6 1 1 1

The relaxation is taking forever so I've checked the output and I've
found several steps where everything seems to be converged (notice I'm
only relaxing out-of-plane lattice vector) but the code keeps running.
An example:

- Energy difference = 4e-7
- Forces acting on atoms (Ry/au):

     atom   1 type  1   force =     0.00000000    0.00000000    0.00085679
     atom   2 type  2   force =     0.00000000    0.00000000   -0.00001779
     atom   3 type  3   force =     0.00000000    0.00000000   -0.00024617
     atom   4 type  3   force =     0.00000000    0.00000000   -0.00034665
     atom   5 type  3   force =     0.00000000    0.00000000   -0.00024617

     Total force =     0.000988     Total SCF correction =     0.000002

          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.84
  -0.00001150   0.00000000   0.00000000         -1.69      0.00      0.00
   0.00000000  -0.00001150   0.00000000          0.00     -1.69      0.00
   0.00000000   0.00000000   0.00000591          0.00      0.00      0.87

Any idea of what's going on?

Thank you very much

Pablo