[Pw_forum] convergence stops after 100 iterations

[manchugh at iitk.ac.in]

Hi Quantum Espresso users

I am trying to perform 'relax' calculations of GaN surface and trying to
optimize the vacuum gap keeping the slab thickness same. One side of the
slab (the N-terminated one) is passivated by pseudohydrogens of charge
0.75. For some values of the vacuum gap, relaxation proceeded
successfully, (got Final energy in 12 scf cycles) but for some other
values of vacuum gap, the 100 iterations in the Ist scf cycle itself are
not sufficient and the following error appears:

convergence NOT achieved in 100 iterations: stopping

I am using  espresso-5.0.2 and my input file is as follows :

&CONTROL
    calculation = "relax",
    title = '1x1x4_7gap'
    verbosity = 'high',
    restart_mode = 'from_scratch',
    prefix = '1x1x4_7'
    pseudo_dir = '/home/.....',
    outdir = '/home/....../',
    tprnfor = .TRUE.,
    etot_conv_thr = 1.0D-04,
    forc_conv_thr = 1.0D-03,
    nstep = 100
 /
&SYSTEM
    ibrav = 0 ,
    celldm(1)= 6.088,
    nat = 19 ,
    ntyp = 3 ,
    ecutwfc = 60.0 ,
    ecutrho = 480.0 ,
    input_dft = 'PBE' ,
    occupations = 'smearing',
    smearing = 'methfessel-paxton' ,
    degauss = 0.05
 /

&ELECTRONS
  diagonalization='cg',
  conv_thr = 1.D-6,
  mixing_mode = 'local-TF',
  mixing_beta = 0.2D0
/

&IONS
  ion_dynamics = 'bfgs'
/

CELL_PARAMETERS {hexagonal}
 1.00  0.00   0.00
 0.50  0.866  0.00
 0.00  0.00   17.886

ATOMIC_SPECIES
  Ga  69.723  Ga.pbe-n-van.UPF
  N   14.006  N.pbe-van_ak.UPF
  H   1.0079  H.pz-vbc_075.UPF

ATOMIC_POSITIONS {crystal}
H   0.3333  0.3333  -0.029195     0  0  0
N   0.3333  0.3333  -0.0111818    0  0  0
Ga  0.000   0.000   0.000         0  0  0
N   0.000   0.000   0.03427272
Ga  0.3333  0.3333  0.04545454
N   0.3333  0.3333  0.07972727
Ga  0.000   0.000   0.09090909
N   0.000   0.000   0.12518181
Ga  0.3333  0.3333  0.13636363
N   0.3333  0.3333  0.17063636
Ga  0.000   0.000   0.18181818
N   0.000   0.000   0.21609090
Ga  0.3333  0.3333  0.22727272
N   0.3333  0.3333  0.26154545
Ga  0.000   0.000   0.27272727
N   0.000   0.000   0.30699999
Ga  0.3333  0.3333  0.31818181
N   0.3333  0.3333  0.35245454
Ga  0.000   0.000   0.36363636

K_POINTS (automatic)
4  4  1  0  0  0


For further information, this is a system with vacuum gap equivalent to 7
GaN unit cells, the system with vacuum gap equivalent to 6 GaN unit cells
worked fine. I am not getting why this problem is occurring only for few
cases?

Any kind of help or suggestions will be very valuable to me.


Thanks
-
Manjusha
Research Scholar
Department of Chemistry
Indian Institute of Technology,Kanpur
India