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

[Dr. Thomas Brumme]

Dear Bin,

I have no clue if it could be done but it is definitely not  
implemented. There are problems:
Firstly, changing the lattice also changes the doping charge per area  
and thus the dipole. Yes, this is calculated self-consistently anyways  
but in the total energy there're parts related to the dipole and the  
gate interacting with themselves - how to take them properly into  
account? One would need to go through the equations related to the  
gate and to stress in order to understand what needs to be done. We  
just assumed that the influence on the lattice due to the doping is  
small - which is certainly not true for all materials. If you find a  
way to implement this, please let me know ;)

Thomas

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

> Dear Thomas,
>
>
>
> Thank you so much for your reply! I redo the calculation with a  
> lower doping level and hide the gate in the barrier as you  
> suggested. The calculation successfully gets converged. However, I  
> have another question that if we relax the lattice constant of 2D  
> materials, e.g. graphene, under the field-effect setup, i.e. the  
> influence of gating on the lattice constant of 2D materials. Is this  
> meaningful? If yes, can we do this by setting calculation =  
> ‘relax-vc’ or by calculating the curve of energy vs. lattice constant?
>
>
>
> Best,
>
> Bin
>
>
>
>
>
> ________________________________
> From: Dr. Thomas Brumme <thomas.brumme at uni-leipzig.de>
> Sent: Monday, September 16, 2019 7:42:59 PM
> To: Quantum ESPRESSO users Forum <users at lists.quantum-espresso.org>;  
> bshaonku at outlook.com <bshaonku at outlook.com>
> Subject: Re: [QE-users] bad convergence in field-effect  
> configuration calculations
>
> 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
>> ======================================================================================