Polar 2D materials and the new Coulomb truncation in QE.

[chrisewolf]

Dear all,

has anyone tested the new compatibility of the new assume_isolated='2D' in QE 6.2.1 and EPW?

I get very nice phonon and electronic bands (for a single layer MgO) but unfortunately the Im electron selfenergy seems fairly high (~2-3 times compared to bulk). Maybe someone could shed some light on critical parameters for surface slabs as converging them is very costly due to the phonon calculations of supercells...

bulk case: (from the school 2018 example with slightly increased convergence parameters)

Code: Select all

# Electron lifetime (meV)
#      ik       ibnd                 E(ibnd)      Im(Sgima)(meV)
        1          1   -0.43879808077095E+01    0.93693811231612E+02
        1          2   -0.13555044640302E+01    0.16520293350459E+03
        1          3   -0.13555044640299E+01    0.16520293350459E+03
        2          1   -0.43793837782571E+01    0.14556183487812E+03
        2          2   -0.13512521215959E+01    0.15663260734296E+03
        2          3   -0.13485880053560E+01    0.17020377483495E+03
        3          1   -0.43615845989129E+01    0.13242126558615E+03
        3          2   -0.13461480999661E+01    0.15583520501622E+03

slab:

Code: Select all

# Electron lifetime (meV)
#      ik       ibnd                 E(ibnd)      Im(Sgima)(meV)
        1         11   -0.34015159446783E+01    0.93072263458128E+04
        1         12   -0.34015159445487E+01    0.93072263458128E+04
        1         13   -0.64756993808455E+00    0.46169269270089E+05
        1         14   -0.64756993808428E+00    0.46169269270089E+05
        1         15   -0.27695066546182E+00    0.18859770839095E+05
        1         16   -0.27695066544299E+00    0.18859770839095E+05
        2         11   -0.34012294161340E+01    0.93712881929632E+04

The slab was calculated on 10^6 random q points. the coarse grid is 12x12x1 for scf, 4x4x1 q points for phonons and 8x8x1 k points for nscf/wannier.

Unfortunately there are no examples for 2D as of yet. If my calculates gives something meaningful in the end I would of course donate the input and results and donate it - sharing is caring

epw.in

Code: Select all

--
&inputepw
  prefix      = 'mgo'
  amass(1) = 24.30500
  amass(2) = 15.99900 
  outdir      = './'
  etf_mem     = 0
  iverbosity  = 0

  system_2d   = .true.

  elph        = .true.

  epbwrite    = .false.
  epbread     = .true.

  epwwrite    = .false.
  epwread     = .true.

  kmaps       = .true.

  wannierize  = .false.
  nbndsub     =  16
  nbndskip    =  0
  num_iter    = 300
  iprint      = 2
  
  !dis_win_max = -9
  !dis_froz_min= -20
  !dis_froz_max= -4
  proj(1)     = 'O:s;p'  
  proj(2)     = 'Mg:s;p'
  wdata(1) = 'bands_plot = .true.'
  wdata(2) = 'begin kpoint_path'
  wdata(3) = 'M 0.50000    0.50000    0.00000  G  0.00000    0.00000    0.00000'
  wdata(4) = 'G 0.00000    0.00000    0.00000  X  0.50000    0.00000    0.00000'
  wdata(5) = 'X 0.50000    0.00000    0.00000  M  0.50000    0.50000    0.00000 '
  wdata(7) = 'end kpoint_path' 
  wdata(8) = 'write_hr=.false.'
  wdata(9) = 'write_hr_diag = .true.'

  elecselfen  = .true.
  phonselfen  = .false.
  a2f         = .false.

  specfun_el   = .false. 
  specfun_ph   = .false.
  wmin_specfun = -4
  wmax_specfun = 1
  nw_specfun   = 101
  
  band_plot   = .false. 
  !lifc       = .true.
  asr_typ     = 'simple'
  lpolar      = .true.
  shortrange  = .true.
  !longrange   = .true.
  prtgkk      = .false.

  efermi_read = .true. 
  fermi_energy =  -4.9385

  parallel_k  = .true.
  parallel_q  = .false.

  fsthick     = 3   ! eV 
  eptemp      = 0.5 ! K 
  degaussw    = 0.01 ! eV

  dvscf_dir   = '../phonon/save'
  filukk      = './mgo.ukk'

  !filqf       = 'meshes/MGXM.dat'
  filkf       = 'meshes/MGXM.dat'

  rand_q      = .true.
  rand_nq     = 1000

  !rand_k      = .true.
  !rand_nk     = 1000000

  !nqf1        = 50
  !nqf2        = 50
  !nqf3        = 1
  
  !nkf1        = 50
  !nkf2        = 50
  !nkf3        = 1

  nk1         = 8
  nk2         = 8
  nk3         = 1

  nq1         = 4
  nq2         = 4
  nq3         = 1
/
  6 cartesian
         0.000000000   0.000000000   0.000000000
         0.000000000   0.250000000   0.000000000
         0.000000000  -0.500000000   0.000000000
         0.250000000   0.250000000   0.000000000
         0.250000000  -0.500000000   0.000000000
        -0.500000000  -0.500000000   0.000000000 

Code: Select all

&CONTROL
 calculation = 'nscf'
 prefix='mgo',
 pseudo_dir = '../../pseudo/',
 outdir='./'
 wf_collect=.true.
 verbosity='high'
/
&SYSTEM
  ibrav       = 6
  celldm(1)   = 7.9689
  celldm(3)   = 6
  nat         = 4
  ntyp        = 2
  ecutwfc     = 120
 assume_isolated = '2D'
/
&ELECTRONS
 diago_full_acc=.true.
 conv_thr=1e-12
/
ATOMIC_SPECIES
  Mg   24.30500  Mg_ONCV_PBE-Chris.upf
   O   15.99900  O_ONCV_PBE-Chris.UPF
ATOMIC_POSITIONS {crystal}
Mg   0.000000000000000   0.000000000000000   0.000000000000000 
Mg   0.500000000000000   0.500000000000000   0.000000000000000 
 O   0.500000000000000   0.000000000000000   0.000000000000000 
 O   0.000000000000000   0.500000000000000   0.000000000000000 
K_POINTS crystal
64
  0.00000000  0.00000000  0.00000000  1.562500e-02 
  0.00000000  0.12500000  0.00000000  1.562500e-02 
  0.00000000  0.25000000  0.00000000  1.562500e-02 
  0.00000000  0.37500000  0.00000000  1.562500e-02 
  0.00000000  0.50000000  0.00000000  1.562500e-02 
  0.00000000  0.62500000  0.00000000  1.562500e-02 
  0.00000000  0.75000000  0.00000000  1.562500e-02 
  0.00000000  0.87500000  0.00000000  1.562500e-02 
  0.12500000  0.00000000  0.00000000  1.562500e-02 
  0.12500000  0.12500000  0.00000000  1.562500e-02 
  0.12500000  0.25000000  0.00000000  1.562500e-02 
  0.12500000  0.37500000  0.00000000  1.562500e-02 
  0.12500000  0.50000000  0.00000000  1.562500e-02 
  0.12500000  0.62500000  0.00000000  1.562500e-02 
  0.12500000  0.75000000  0.00000000  1.562500e-02 
  0.12500000  0.87500000  0.00000000  1.562500e-02 
  0.25000000  0.00000000  0.00000000  1.562500e-02 
  0.25000000  0.12500000  0.00000000  1.562500e-02 
  0.25000000  0.25000000  0.00000000  1.562500e-02 
  0.25000000  0.37500000  0.00000000  1.562500e-02 
  0.25000000  0.50000000  0.00000000  1.562500e-02 
  0.25000000  0.62500000  0.00000000  1.562500e-02 
  0.25000000  0.75000000  0.00000000  1.562500e-02 
  0.25000000  0.87500000  0.00000000  1.562500e-02 
  0.37500000  0.00000000  0.00000000  1.562500e-02 
  0.37500000  0.12500000  0.00000000  1.562500e-02 
  0.37500000  0.25000000  0.00000000  1.562500e-02 
  0.37500000  0.37500000  0.00000000  1.562500e-02 
  0.37500000  0.50000000  0.00000000  1.562500e-02 
  0.37500000  0.62500000  0.00000000  1.562500e-02 
  0.37500000  0.75000000  0.00000000  1.562500e-02 
  0.37500000  0.87500000  0.00000000  1.562500e-02 
  0.50000000  0.00000000  0.00000000  1.562500e-02 
  0.50000000  0.12500000  0.00000000  1.562500e-02 
  0.50000000  0.25000000  0.00000000  1.562500e-02 
  0.50000000  0.37500000  0.00000000  1.562500e-02 
  0.50000000  0.50000000  0.00000000  1.562500e-02 
  0.50000000  0.62500000  0.00000000  1.562500e-02 
  0.50000000  0.75000000  0.00000000  1.562500e-02 
  0.50000000  0.87500000  0.00000000  1.562500e-02 
  0.62500000  0.00000000  0.00000000  1.562500e-02 
  0.62500000  0.12500000  0.00000000  1.562500e-02 
  0.62500000  0.25000000  0.00000000  1.562500e-02 
  0.62500000  0.37500000  0.00000000  1.562500e-02 
  0.62500000  0.50000000  0.00000000  1.562500e-02 
  0.62500000  0.62500000  0.00000000  1.562500e-02 
  0.62500000  0.75000000  0.00000000  1.562500e-02 
  0.62500000  0.87500000  0.00000000  1.562500e-02 
  0.75000000  0.00000000  0.00000000  1.562500e-02 
  0.75000000  0.12500000  0.00000000  1.562500e-02 
  0.75000000  0.25000000  0.00000000  1.562500e-02 
  0.75000000  0.37500000  0.00000000  1.562500e-02 
  0.75000000  0.50000000  0.00000000  1.562500e-02 
  0.75000000  0.62500000  0.00000000  1.562500e-02 
  0.75000000  0.75000000  0.00000000  1.562500e-02 
  0.75000000  0.87500000  0.00000000  1.562500e-02 
  0.87500000  0.00000000  0.00000000  1.562500e-02 
  0.87500000  0.12500000  0.00000000  1.562500e-02 
  0.87500000  0.25000000  0.00000000  1.562500e-02 
  0.87500000  0.37500000  0.00000000  1.562500e-02 
  0.87500000  0.50000000  0.00000000  1.562500e-02 
  0.87500000  0.62500000  0.00000000  1.562500e-02 
  0.87500000  0.75000000  0.00000000  1.562500e-02 
  0.87500000  0.87500000  0.00000000  1.562500e-02 

[andreyl]

Hi!

As far as I know, assume_isolated='2D' is unusable with EPW right now.
It, most probably, breaks at the point where the potential change with phonon perturbation is written in QE ph.x calculation. The resulting electron phonon coupling elements are enormous in EPW: factor ~100 (or higher) from expected, I wasn't able to determine the exact factor and not sure it is possible to do without examining the code carefully. Since |g| is incorrect and high all the values it enters are as well.

If you find out anything about that, it would be super great, if you post it somewhere here.

Best wishes,
Andrei

[chrisewolf]

Hi Andrei,

good (bad?) to know you experienced a similar behaviour, means it is not my fault entirely

I will check if the dvscf differ significantly and I am running the job again using dipfield instead (which apparently does not work properly in ph.x according to the mailing list but oh well...)!

I will keep you posted!

Chris

[sponce]

Hello Chris,

Indeed 2D materials are not officially supported in EPW (it is quite high up in my to do list).

I can see a number of problems. One of them is the fact that the g should diverge in polar materials for q-->0 but in the case of 2D materials it will go to a finite value.

The current analytic treatment of g for small q only works for 3D materials.

You should plot the g (using the prtgkk feature) to see this.

You can try to use lpolar=false and use a dense coarse grid. This should help.
You can also try to code the correct 2D analytical behavior for small q of the el-ph matrix elements

Best wishes,
Samuel