[CP2K-user] What is a "charged system" in the context of a polarization calculation?

JN436 sf1... at gmail.com
Tue Apr 6 13:23:56 UTC 2021

I calculated dipole moments without specifying a "reference point" and 
would now like to find out if I need to redo them with a reference point.  

According to a previous post 
(https://lists.cp2k.org/archives/cp2k-user/2018-July/010536.html), my 
approach should be correct if my system is not charged.  This may be a 
simple concept but I don't understand what it means.

If the system is 32 H2O (for example), then it is charge-neutral.  What if 
some of the water dissociates or I add a negatively charged ion and a 
positively charged ion?  The system will still be charge-neutral overall, 
but locally, it won't be.  Can anyone explain whether to use a reference 
point and which one would be correct in such a situation?  Thank you.  
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