[QE-users] Dipole correction: should there be a "dip" in the electrostatic potential?

[Christoph Wolf]

Dear all,

I am (again...) fighting with the dipole correction. I have a system
consisting of Ag and MgO centered in the cell around z=0.5

ATOMIC_POSITIONS (crystal)
Ag       0.000000000   0.000000000   0.323271580
Ag       0.000000000   0.500000000   0.385512833
....
Mg      -0.000000000   0.000000000   0.721978968
O        0.500000000   0.000000000   0.723372273
O       -0.000000000   0.500000000   0.723371161
Mg       0.500000000   0.500000000   0.721975961

Then I apply tefield and dipfield with

  eamp        = 0.00
  edir        = 3
  emaxpos     = 0.95
  eopreg      = 0.10

which means my system has no discontinuity anywhere in the slab and the
field is "physical" between 0.95+0.1-1=0.05 and 0.95 of the z direction.

The SCF cycle converges but the pot_num=11 never becomes horizontal, rather
it has a very small slope. I tried to increase the dipole length to see if
that makes a difference but apparently it does not help either. With above
input the potential amplitude at scf convergence is

     Adding external electric field

     Computed dipole along edir(3) :
        Elec. dipole          0.4894 Ry au,          1.2440 Debye
        Ion. dipole           0.4902 Ry au,          1.2460 Debye
        Dipole                0.2274 Ry au,          0.5779 Debye
        Dipole field          0.0008 Ry au,

        Potential amp.       -0.0855 Ry
        Total length         55.5874 bohr


Is there a scenario where the dipole correction could fail if the potential
is "almost flat" in the vacuum region? If I run the same with VASP I get a
small "jump" in the potential making it flat in the vacuum region, so I was
wondering why this does not happen (probably different approach in PWSCF)?

Input and resulting potential is attached - any help is welcome!

Thanks in advance and with best wishes,
Chris
-- 
Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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