[Pw_forum] Trying to clarify the dipole correction (again!)

[Stefano de Gironcoli]

Dear J.J. Ramsey

     at variance with what is defined in Bengtsson's paper the dipole 
potential is not discontinuous  in QE but is defined by periodically 
alternating an increasing slope (associate to the "physical" electric 
field present in the cell) and a  rapidly decreasing slope that should 
be located in a region of negligibly small charge density. This is done 
to avoid too sharp features in the calculation. The definition is also 
generalized so that the region where the dipole correction operates can 
be centered in an arbitrary point of the cell and not just in the center 
of it as defined in the paper.
    the regression region (the region with a rapidly decreasing slope) 
occupies a fraction eopreg of the cell size while the maximum of the 
increasing slope occurs at a fraction emaxpos.
   so if emaxpos = 0.45 and eopreg=0.1 the region where the correction 
is physically meaningful goes from -45% to +45% of the cell and the 
regression  region  (that should contains no charge density)  occupies 
the central 10% of the cell. This is a good setting if you center your 
slab around the origin.
   if instead emaxpos=0.95 and eopreg=0.1 the the dipole correction is 
applied for +5% to +95% of the cell while in the rest one have the
regression region. this is OK if you put your slab in the center of the 
cell.

  Stefano de Gironcoli - SISSA and DEMOCRITOS


J. J. Ramsey wrote:
> In the reference on the dipole correction used in QE, L. Bengtsson, PRB 59, 12301 (1999), the dipole potential is given as
>
> V_dip(z) = 4*pi*m*(z/z_m - 0.5), 0 < z < z_m
>
> where z_m is the height of the box, and there is a discontinuity in the potential at z = 0,z_m. I'm not sure how this fits into the QE documentation on the dipole correction. Is V_dip in QE supposed to be something like this?:
>
> V_dip(z) = 4*pi*m*[z/z_m - (emaxpos + eopreg -1)], emaxpos + eopreg -1 < z/z_m < emaxpos;
>          = something with a steep but finite slope, otherwise
>
> My guess is probably wrong, but the explanations in the documentation of the roles of emaxpos and eopreg appear to be tied to the case where eamp  is nonzero. I'm not sure if the documentation for the dipfield variable is supposed to indicate that the actual slope of the potential is supposed to be eamp + 4*pi*m/z_m.
>
> University of Akron
> Civil Engineering (!) Dept.
>
>
>       
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