[Pw_forum] SCF calculation don' converge for surface slab

[patriot pershing]

Dear quantum espresso user's
i have tried to run pw calculaion for a 44 atoms surface slab model of ZnO
using the next input file;

&CONTROL
                       title = ZnO ,
                 calculation = 'relax' ,
                 max_seconds = 190000 ,
                restart_mode = 'from_scratch' ,
                      outdir = 'C:/cygwin/home/pbe/' ,
                      wfcdir = 'C:/cygwin/home/pbe/' ,
                  pseudo_dir = 'C:/cygwin/home/pbe/' ,
                      prefix = 'ZnO' ,
                 lkpoint_dir = .false. ,
                     disk_io = 'high' ,
               etot_conv_thr = 1.0D-6 ,
               forc_conv_thr = 1.0D-4 ,
                     tstress = .true. ,
                     tprnfor = .true. ,
                     tefield = .true. ,
                    dipfield = .true. ,
                    lelfield = .false. ,
                      lberry = .false. ,
 /
 &SYSTEM
                       ibrav = 4,
                   celldm(1) = 12.358357076,
                   celldm(3) = 3.200,
                         nat = 44,
                        ntyp = 3,
                     ecutwfc = 50 ,
                     ecutrho = 200 ,
                       nosym = .false. ,
                  tot_charge = 0.000000,
                 occupations = 'smearing' ,
                     degauss = 0.001 ,
                    smearing = 'gaussian' ,
                  lda_plus_u = .false. ,
 /
 &ELECTRONS
            electron_maxstep = 50,
                    conv_thr = 1.0D-5 ,
                 mixing_beta = 0.7 ,
 /
 &IONS
                ion_dynamics = 'bfgs' ,
 /
ATOMIC_SPECIES
   Zn   65.39000  zn205.cpi.UPF
    O   15.99900  onrr.cpi.UPF
    H    1.00740  hh.cpi.UPF
ATOMIC_POSITIONS angstrom
    H      0.000000000    0.000000000    0.242600000    0  0  0
    O      0.000000000    1.887819846    2.595050188    0  0  0
   Zn      0.000000000    0.000000000    1.972653799    0  0  0
   Zn      0.000000000    1.887819846    4.567759359    0  0  0
    H      3.269799948    0.000000000    0.242600000    0  0  0
    O      3.269799948    1.887819846    2.595050188    0  0  0
   Zn      3.269799948    0.000000000    1.972653799    0  0  0
   Zn      3.269799948    1.887819846    4.567759359    0  0  0
    H     -1.634899974    2.831729820    0.242600000    0  0  0
    O     -1.634899974    4.719549666    2.595050188    0  0  0
   Zn     -1.634899974    2.831729820    1.972653799    0  0  0
   Zn     -1.634899974    4.719549666    4.567759359    0  0  0
    H      1.634899974    2.831729820    0.242600000    0  0  0
    O      1.634899974    4.719549666    2.595050188    0  0  0
   Zn      1.634899974    2.831729820    1.972653799    0  0  0
   Zn      1.634899974    4.719549666    4.567759359    0  0  0
    O      0.000000000    0.000000000    5.189899921
    O      0.000000000    1.887819846    7.784950109
   Zn      0.000000000    0.000000000    7.162553721
   Zn      0.000000000    1.887819846    9.757659281
    O      3.269799948    0.000000000    5.189899921
    O      3.269799948    1.887819846    7.784950109
   Zn      3.269799948    0.000000000    7.162553721
   Zn      3.269799948    1.887819846    9.757659281
    O     -1.634899974    2.831729820    5.189899921
    O     -1.634899974    4.719549666    7.784950109
   Zn     -1.634899974    2.831729820    7.162553721
   Zn     -1.634899974    4.719549666    9.757659281
    O      1.634899974    2.831729820    5.189899921
    O      1.634899974    4.719549666    7.784950109
   Zn      1.634899974    2.831729820    7.162553721
   Zn      1.634899974    4.719549666    9.757659281
    O      0.000000000    0.000000000   10.379799843
    O      0.000000000    1.887819846   12.974850030
   Zn      0.000000000    0.000000000   12.352453642
    O      3.269799948    0.000000000   10.379799843
    O      3.269799948    1.887819846   12.974850030
   Zn      3.269799948    0.000000000   12.352453642
    O     -1.634899974    2.831729820   10.379799843
    O     -1.634899974    4.719549666   12.974850030
   Zn     -1.634899974    2.831729820   12.352453642
    O      1.634899974    2.831729820   10.379799843
    O      1.634899974    4.719549666   12.974850030
   Zn      1.634899974    2.831729820   12.352453642
K_POINTS automatic
  4 4 1   0 0 0

however the convergence was failed
i have tried o use different K points mesh such as 3x3x1 or 5x5x1 but the
problem is the same. i have changed the vacum region from 10 to 15 A, i
have used the dipole correction also but the convergence problem was not
resolved
so if any one have a solution le me know
best regards
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