[Pw_forum] charhe simulation

[Axel Kohlmeyer]

On Sat, 2009-06-06 at 00:20 -0700, ali kazempour wrote:
> Dear all
> I have a question about charge simulation.

> In common DFT simulation with neutral charge we have Ewald sum that
>  describe charge-charge interaction . This sum is not problematic . But

the ewald sum is a _method_ to efficiently compute (or rather
approximate) the _coulomb contribution_ to the total energy 
per unitcell in a crystal.

>  when we simulate for example ionization energy, we have to use
>  correction like makov-payne . In both case we have charge -charge
>  interaction . What is the differences between two case?

if you take out an electron, you will have a charged unit cell,
and due to periodic boundary conditions, this will happen in
_every_ unitcell. this is effectively an unphysical situation 
and such a crystal is not stable. however, you can still do 
calculations of such systems, if you ignore the g=0 term in 
the ewald sum. this will lead to an error in the total energy, 
but it is independent of the positions of the atoms or ions 
and thus the forces will still be correct. 

cheers,
   axel.

> Thanks a lot
> 
> Ali Kazempour
> Physics department, Isfahan University of Technology
> 84156 Isfahan, Iran.            Tel-1:  +98 311 391 3733
> Fax: +98 311 391 2376      Tel-2:  +98 311 391 2375
> 
> 
>       
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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
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