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

[Stefano de Gironcoli]

J. J. Ramsey wrote:
> ----- 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.
>   
yes
>>    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.
>   
or centered around the origin like in the the Al(001) surface of 
example03..
remember that we have periodic boundary conditions and the origin does 
not have any special meaning.

stefano