[CP2K:1759] Re: Total dipole via LOCALIZATION - bug?

Fawzi Mohamed fa... at gmx.ch
Wed Feb 4 15:40:27 UTC 2009

On 3-feb-09, at 13:35, Fawzi Mohamed wrote:

> On 3-feb-09, at 12:22, Juerg Hutter wrote:
>> Hi Fawzi
>>> The thing my code did and is not done now is simply to remove jumps
>>> by
>>> L/2*pi, and output also the finite derivative of the dipole, this
>>> could also be added to the present code as an option.
>>> If you use the total dipole of a periodic system setting the
>>> reference
>>> to COAC is not so useful, and does not allow you to remove the jumps
>>> easily.
>> Can you explain why the removal of jumps depends on the reference
>> point?
>> If you use COAC there is only one non-zero term, meaning there is  
>> also
>> one term that can jump. For all other cases both contributions can
>> jump independently.
>> Juerg
> you are right, indeed the simple removal of jumps works also with  
> I was thinking that using an oscillating reference point might
> introduce a spurious signal.
> Without the berry phase as long at the system is neutral the reference
> point has no importance, but if one treats only the electrons, the
> system is charged, and one has to take care how the ions+electrons
> system if brought back together (only for the whole system there is no
> problem).
> Using the berry phase the thing should be the same, but I already  
> flip-
> flopped a couple of times on it.
> I would say that the thing is not immediately obvious for systems
> distributed on the whole cell, but it might well be that you are right
> (it makes sense for neutral systems).
> Fawzi

following up my own message ;)
yes indeed the dipole is always independent from the reference point  
for neutral systems (as it should be).
So Juerg you were right that COAC is a good choice to minimize the  
jumps for neutral systems.

Victoria, using periodic total dipole you should have the same results  
(maybe even better, given joost bugfixes).
The only difference is that now the last column does not remove the  
jumps in the dipole, whereas the version you had did (these are jumps  
due to one electron jumping from one side to the other of the cell).
They are easy to remove in post processing (in reduced units), but  
would also be trivial to add (a call to pbc for the delta_dipole when  
do_berry is true).


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