[Pw_forum] Problem in SCF convergence with larger unit cells

[reza vatan]

Dear all,

I'm trying to optimize the atomic structure of GaP using QE. When I use a
unit cell only containing two atoms, I have no problem in SCF convergence
and the structure get optimized in the first step. But when I make my unit
cell bigger (let's say a unit cell with 64 atoms) just by repeating my
smaller optimized unit cell in real space and run relax calculation, the
SCF calculation never get converged. Does anyone know where the problem is?

Following is my input:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
&CONTROL
  calculation = "relax",
  dt          = 30.D0,
  prefix = 'gap',
  pseudo_dir = './',
  outdir='./',
/
 &system
    ibrav= 6, celldm(1) =20.52, celldm(3) =1, nat=  64, ntyp= 2,
    ecutwfc = 12.0,
  degauss     = 0.05D0,
  occupations = "fixed",
  smearing    = "methfessel-paxton",
 /
 &ELECTRONS
  conv_thr    = 1.D-7,
  mixing_beta = 0.3D0,
  electron_maxstep= 200,
/
&IONS
  pot_extrapolation = "second_order",
  wfc_extrapolation = "second_order",
 /
ATOMIC_SPECIES
Ga  69.72  Ga.blyp-d-hgh.UPF
P  30.97  P.blyp-hgh.UPF
ATOMIC_POSITIONS (angstrom)
Ga          2.7146780000          2.7146780000          0.0000000000
Ga          0.0000000000          0.0000000000          0.0000000000
Ga          2.7146780000          0.0000000000          2.7146780000
Ga          0.0000000000          2.7146780000          2.7146780000
P          4.0720170000          4.0720170000          1.3573390000
P          1.3573390000          1.3573390000          1.3573390000
P          4.0720170000          1.3573390000          4.0720170000
P          1.3573390000          4.0720170000          4.0720170000
Ga          2.7146780000          2.7146780000          5.4293560000
Ga          0.0000000000          0.0000000000          5.4293560000
Ga          2.7146780000          0.0000000000          8.1440340000
Ga          0.0000000000          2.7146780000          8.1440340000
P          4.0720170000          4.0720170000          6.7866950000
P          1.3573390000          1.3573390000          6.7866950000
P          4.0720170000          1.3573390000          9.5013730000
P          1.3573390000          4.0720170000          9.5013730000
Ga          2.7146780000          8.1440340000          0.0000000000
Ga          0.0000000000          5.4293560000          0.0000000000
Ga          2.7146780000          5.4293560000          2.7146780000
Ga          0.0000000000          8.1440340000          2.7146780000
P          4.0720170000          9.5013730000          1.3573390000
P          1.3573390000          6.7866950000          1.3573390000
P          4.0720170000          6.7866950000          4.0720170000
P          1.3573390000          9.5013730000          4.0720170000
Ga          2.7146780000          8.1440340000          5.4293560000
Ga          0.0000000000          5.4293560000          5.4293560000
Ga          2.7146780000          5.4293560000          8.1440340000
Ga          0.0000000000          8.1440340000          8.1440340000
P          4.0720170000          9.5013730000          6.7866950000
P          1.3573390000          6.7866950000          6.7866950000
P          4.0720170000          6.7866950000          9.5013730000
P          1.3573390000          9.5013730000          9.5013730000
Ga          8.1440340000          2.7146780000          0.0000000000
Ga          5.4293560000          0.0000000000          0.0000000000
Ga          8.1440340000          0.0000000000          2.7146780000
Ga          5.4293560000          2.7146780000          2.7146780000
P          9.5013730000          4.0720170000          1.3573390000
P          6.7866950000          1.3573390000          1.3573390000
P          9.5013730000          1.3573390000          4.0720170000
P          6.7866950000          4.0720170000          4.0720170000
Ga          8.1440340000          2.7146780000          5.4293560000
Ga          5.4293560000          0.0000000000          5.4293560000
Ga          8.1440340000          0.0000000000          8.1440340000
Ga          5.4293560000          2.7146780000          8.1440340000
P          9.5013730000          4.0720170000          6.7866950000
P          6.7866950000          1.3573390000          6.7866950000
P          9.5013730000          1.3573390000          9.5013730000
P          6.7866950000          4.0720170000          9.5013730000
Ga          8.1440340000          8.1440340000          0.0000000000
Ga          5.4293560000          5.4293560000          0.0000000000
Ga          8.1440340000          5.4293560000          2.7146780000
Ga          5.4293560000          8.1440340000          2.7146780000
P          9.5013730000          9.5013730000          1.3573390000
P          6.7866950000          6.7866950000          1.3573390000
P          9.5013730000          6.7866950000          4.0720170000
P          6.7866950000          9.5013730000          4.0720170000
Ga          8.1440340000          8.1440340000          5.4293560000
Ga          5.4293560000          5.4293560000          5.4293560000
Ga          8.1440340000          5.4293560000          8.1440340000
Ga          5.4293560000          8.1440340000          8.1440340000
P          9.5013730000          9.5013730000          6.7866950000
P          6.7866950000          6.7866950000          6.7866950000
P          9.5013730000          6.7866950000          9.5013730000
P          6.7866950000          9.5013730000          9.5013730000
K_POINTS {automatic}
2 2 2 0 0 0
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Thanks in advance.

Regards,
Reza Vatan
School of Electrical, Computer and Energy Engineering
Arizona State University
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