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

[J. J. Ramsey]

----- Original Message ----

> From: Stefano de Gironcoli <degironc at sissa.it>
> To: PWSCF Forum <pw_forum at pwscf.org>
> Sent: Thursday, August 13, 2009 11:59:14 AM
> Subject: Re: [Pw_forum] Trying to clarify the dipole correction (again!)
> 
> 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.

In other words, the discontinuity in Bengtsson's paper, where the slope is infinite, is replaced by a large but finite slope.

>    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.

I gather you mean the case where the slab is near the bottom of the unit cell.