[CPMD-list] background charge

[Axel Kohlmeyer]

On Sat, 14 Apr 2007, Weijie HUA wrote:

WH> Dear All,
WH> I'd like to ask if CPMD can do calculations on systems added with
WH> background point charges.

in principle, yes. but it is usually not a good idea,
since you can run into electron 'spillout'. unlike with
gaussian (i.e. atomic centered) basis sets, where the
electrons are confined to the area covered by the basis
functions, with a plane wave basis, electrons can 'go
everywhere' in the simulation cell.

WH> I have checked the manual and original pipemails, but didn't find
WH> topics on background charges. However,I found many discussions on
WH> EXTERNAL POTENTIAL which makes it possible to add an external
WH> electrostatic field. I'd like to ask can this keyword be used to add
WH> background point charges?  If possible, how?

depending on what kind of problem you want to investigate
(the more you tell us about what you plan to do, the better
we can help you!), looking into one of the QM/MM interfaces
might be an alternative (they use essentially the same mechanism
as the EXTERNAL POTENTIAL keyword, but have additional precautions
against spillout). you'd define your point charge field as classical
system and then the QM part as usual...

cheers,
  axel.

WH> 
WH> Many thanks.
WH> 
WH> Best regards,
WH> Weijie	
WH> 
WH> 
WH> 
WH> ----
WH> Weijie HUA, Ph. D. candidate
WH> Institute of Theoretical and Computational Chemistry,
WH> Key Laboratory of Mesoscopic Chemistry,
WH> School of Chemistry and Chemical Engineering,
WH> Nanjing University,Nanjing 210093,
WH> P.R.China
WH> Email:huawj at itcc.nju.edu.cn
WH> 2007-04-14
WH> 
WH> 
WH> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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