[Pw_forum] Relaxation of AlN surface

[Somesh Kumar Bhattacharya]

Dear All,

I am a new to PWSCF.

I am trying to relax a surface slab of AlN which has 32 atoms.
However, the system is not achieving convergence.

The input file is  :

&control
    calculation = 'relax'
    restart_mode='from_scratch',
    prefix='AlN',
    nstep = 100,
    tprnfor = .true.,
    etot_conv_thr = 1.d-8,
    pseudo_dir = '~/QE/espresso-4.0.3/pseudo/',
    outdir='.'
/
 &system
    ibrav=  4, celldm(1) = 11.71623992919922,
    celldm(3) = 4.513,nat=32, ntyp= 2,
    ecutwfc =30.0, ecutrho = 400, nspin =1,
 /
 &electrons
    electron_maxstep=150,
    conv_thr = 1.D-6,
    mixing_mode = 'local-TF',
    mixing_beta = 0.2,
/
 &ions
     ion_dynamics='bfgs',
     phase_space = 'full'
/
ATOMIC_SPECIES
Al  26.98154  Al.pw91-n-van.UPF
N   14.00674  N.pw91-van_ak.UPF
ATOMIC_POSITIONS {angstrom}
Al  1.549999937  4.474454315  14.919999921    1 1 1
Al  1.549999937  4.474454315  9.919999843     0 0 0
Al -0.000000015  7.159132936  14.919999921    1 1 1
Al -0.000000015  7.159132936  9.919999843     0 0 0
Al  4.649999842  4.474454315  14.919999921    1 1 1
Al  4.649999842  4.474454315  9.919999843     0 0 0
Al  3.099999890  7.159132936  14.919999921    1 1 1
Al  3.099999890  7.159132936  9.919999843     0 0 0
N   1.549999937  4.474454315  6.818997379     0 0 0
N   1.549999937  4.474454315 11.778997301     0 0 0
N  -0.000000015  7.159132936  6.818997379     0 0 0
N  -0.000000015  7.159132936 11.778997301     0 0 0
N   4.649999842  4.474454315  6.818997379     0 0 0
N   4.649999842  4.474454315 11.778997301     0 0 0
N   3.099999890  7.159132936  6.818997379     0 0 0
N   3.099999890  7.159132936 11.778997301     0 0 0
Al  3.099991178  3.579566454  7.439985857     0 0 0
Al  3.099991178  3.579566454 12.399985779     0 0 0
Al  1.549991226  6.264245075  7.439985857     0 0 0
Al  1.549991226  6.264245075 12.399985779     0 0 0
Al  6.199991083  3.579566454  7.439985857     0 0 0
Al  6.199991083  3.579566454 12.399985779     0 0 0
Al  4.649991131  6.264245075  7.439985857     0 0 0
Al  4.649991131  6.264245075 12.399985779     0 0 0
N   3.099991178  3.579566454  9.298983315     0 0 0
N   3.099991178  3.579566454 14.258983237     1 1 1
N   1.549991226  6.264245075  9.298983315     0 0 0
N   1.549991226  6.264245075 14.258983237     1 1 1
N   6.199991083  3.579566454  9.298983315     0 0 0
N   6.199991083  3.579566454 14.258983237     1 1 1
N   4.649991131  6.264245075  9.298983315     0 0 0
N   4.649991131  6.264245075 14.258983237     1 1 1
K_POINTS {GAMMA}


I am allowing only the top two layers to relax.

Using --- grep scf < *.out   ...... the output is the following :

     estimated scf accuracy    <    21.54311123 Ry
     estimated scf accuracy    <    16.91216138 Ry
     estimated scf accuracy    <     6.80358058 Ry
     estimated scf accuracy    <    10.02440462 Ry
     estimated scf accuracy    <     5.42211905 Ry
     estimated scf accuracy    <     3.98570225 Ry
     estimated scf accuracy    <     3.63368556 Ry
     estimated scf accuracy    <     2.80935163 Ry
     estimated scf accuracy    <     2.18832175 Ry
     estimated scf accuracy    <     1.88438498 Ry
     estimated scf accuracy    <     1.34984265 Ry
     estimated scf accuracy    <     1.15892961 Ry
     estimated scf accuracy    <     1.08983405 Ry
     estimated scf accuracy    <    11.86322500 Ry
     estimated scf accuracy    <     9.08655092 Ry
     estimated scf accuracy    <    10.92075279 Ry
     estimated scf accuracy    <    11.43007849 Ry
      .................

The system never attain convergence.

Can anyone kindly suggest the possible source of error on my part.

Regards,
Somesh
...............................
Somesh Kr. Bhattacharya
Post Doctoral Fellow
Room No. 263,
Leonardo Building,
The Abdus Salam International Centre for Theoretical Physics
Strada Costiera, 11
I-34014 Trieste
Italy
Phone: +39-040-2240399