[QE-users] bad convergence in field-effect configuration calculations

[Dr. Thomas Brumme]

Dear Bin,

I can't check your input thoroughly as I'm on vacation, but there are  
2 things I noticed:

1. The doping seems quite high - maybe you're close to the van Hove  
singularity which could explain problems. Can you try with less doping  
and then - if this converges - increase the doping till you get  
problems? And plot the total potential - maybe the charge is already  
spilling into the vacuum?! This is connected to the second problem:
2. It can help if you "hide" the gate in the barrier. A typical setup  
I used recently:
  zgate        = 0.589,
  emaxpos      = 0.59,
  eopreg       = 0.01,
  block        = .true.,
  block_1      = 0.50,
  block_2      = 0.60,
  block_height = 2.50,
and the system is below 0.5... E.g. for your case, try
    zgate = 0.1311111
    block = .true.
    block_1 = 0.13
    block_2 = 0.23
    block_height = 2.5
    edir = 3
    emaxpos = 0.13
    eopreg = 0.01
The barrier starts with the dipole correction and the gate is right  
behind the dipole... Compare with Fig. 2 of the PRB (2014). Also note  
that 2.5 Ry for the barrier height is an arbitrarily chosen value - I  
noticed other people now also use 2.5 Ry even if 2 Ry or even 1.5 Ry  
should be enough. Yet, the higher the doping, the higher the dipole  
correction the higher the barrier (if you "hide" everything in the  
barrier)...

Regards & Greeting from the lake Garda in Italy

Thomas

Zitat von Bin Shao <bshaonku at outlook.com>:

> Dear all,
>
> I would like to learn how to set up calculations of field-effect  
> configuration in quantum espresso. I read the paper "T. Brumme, M.  
> Calandra, F. Mauri; PRB 89, 245406 (2014)." and started with  
> graphene as an example. The input file is as follows. During the scf  
> calculation, there are a lot of warnings like " c_bands:  1  
> eigenvalues not converged". And the calculation could not get  
> converged after 800 iterations, could anyone help me with the input  
> file? Thanks in advance!
>
> Best regards,
> Bin
>
> ========================================================================
> &CONTROL
>   calculation = 'scf'
>   etot_conv_thr =   2.0000000000d-05
>   forc_conv_thr =   1.0000000000d-04
>   outdir = './out/'
>   prefix = 'aiida'
>   pseudo_dir = './pseudo/'
>   tprnfor = .true.
>   tstress = .true.
>   gate = .true.
>   dipfield = .true.
>   tefield = .true.
>   verbosity = 'high'
> /
> &SYSTEM
>   degauss =   0.02
>   ecutrho =   500
>   ecutwfc =   45
>   ibrav = 0
>   nat = 2
>   ntyp = 1
>   occupations = 'smearing'
>   smearing = 'cold'
>   tot_charge = -0.2
>   zgate = 0.1
>   block = .true.
>   block_1 = 0.13
>   block_2 = 0.23
>   block_height = 2.5
>   edir = 3
>   emaxpos = 0.005
>   eopreg = 0.01
> /
> &ELECTRONS
>   conv_thr =   1.0000000000d-9
>   electron_maxstep = 800
>   mixing_beta =  0.4
>   mixing_mode = 'local-TF'
> /
> ATOMIC_SPECIES
> C      12.011 C.pbe-n-kjpaw_psl.1.0.0.UPF
> ATOMIC_POSITIONS crystal
> C            0.0000000000       0.0000000000       0.3500000000
> C            0.3333333000       0.6666667000       0.3500000000
> K_POINTS automatic
> 65 65 1 0 0 0
> CELL_PARAMETERS angstrom
>       2.4638000000       0.0000000000       0.0000000000
>      -1.2319000000       2.1336508000       0.0000000000
>       0.0000000000       0.0000000000      35.0000000000
> ======================================================================================