[MMTK] H+ in water

[Konrad Hinsen]

On Mar 20, 2007, at 16:32, Davide Ricci wrote:

> about this....you says that Ewald summation adds an uniform charge  
> background to the system, so that the total charge of the system is  
> zero, but you still have a proton in water for example. Is this  
> done in an automatic way ? Or we have to specify that we need a  
> background charge to neutralize the total charge ?

The background charge density is added automatically, without this  
the total energy would be infinite.

If you look at a description of Ewald summation, you will see that a  
part of the charge density is Fourier-transformed. The k=0 term of  
that Fourier transform, which corresponds to the total charge, is  
then not used in the summation. Physically this is equivalent to  
adding a uniform background charge density that neutralizes the total  
charge of the system.

Konrad.
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Konrad Hinsen
Centre de Biophysique Moléculaire, CNRS Orléans
Synchrotron Soleil - Division Expériences
Saint Aubin - BP 48
91192 Gif sur Yvette Cedex, France
Tel. +33-1 69 35 97 15
E-Mail: hinsen at cnrs-orleans.fr
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