[QE-users] scf not converging with saw-like potential

[Maciej Szary]

Dear All,

I've been practicing some basic calculations with an external electric 
field added. I was attempting to calculate the relaxation of 6% strained 
silicene for amplitudes between -1 and 1 V/A (eamp). Runs with eamp 
between -1 and 0.3 V/A were successful, however, anything above did not 
converge. I've tried higher ecutwfc and ecutrho (up to 100 and 100 Ry, 
respectively), I've changed the k-point mesh, I've used mixing_beta 
between 0.1-0.7, I've changed the diagonalization method and the 
mixing_mode, I've tried different pseudopotentials. Still, nothing 
helped. Reducing london_rcut allowed the conversion for 0.4 V/A, 
however, it had no effect for 0.5 V/A and above.

I don't understand why, but the value of eamp seems to be the problem. 
If I reduce eamp, scf will converge, and more perplexingly if I change 
the sign of the amplitude, scf will also converge. My input for 0.5 V/A 
is given below.

Regards,
Maciej Szary
PhD student,
Institute of Physics,
Poznan University of Technology


  &control
     calculation='relax'
     prefix='T1',
     pseudo_dir='$pseudo_dir'
     outdir = 'files/',
     wf_collect=.true.
     etot_conv_thr=1.0D-5
     forc_conv_thr=1.0D-4
     tefield=.true.
     dipfield=.true.
  /
&system
     ibrav=  4,
     a=3.8448
     c=30
     nat=  2,
     ntyp= 1,
     ecutwfc = 55,
     ecutrho=550,
     occupations= 'smearing',
     smearing='gauss',
     degauss= 0.0001
     assume_isolated = '2D'

     lspinorb=.true.,
     noncolin=.true.,
     starting_spin_angle=.true.

     vdw_corr='DFT-D'
     london_s6=0.75
     london_rcut=15.00

     edir=3
     eamp=0.009723453
     eopreg=0.05
     emaxpos=0.85
/
  &electrons
     electron_maxstep = 150
     conv_thr= 1.0d-7
     diagonalization='cg'
     mixing_beta=0.5
     mixing_mode='local-TF'
     scf_must_converge=.false.
  /
  &ions
     ion_dynamics='bfgs'
  /
ATOMIC_SPECIES
     Si  28.0855  Si.rel-pbe-n-nc.UPF
ATOMIC_POSITIONS crystal
Si      -0.333333300   0.333333330   0.579899915    0   0   1
Si       0.000000000   0.000000000   0.562100085    0   0   1
K_POINTS automatic
12 12 1 0 0 0
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