[CP2K-user] [CP2K:21425] Help with Charge Compensation and Site Selection in NASICON Structure (Zr → Mg Substitution)

[Sam Hesamie]

Dear all [image: 12.PNG]
I'm working with a *NASICON-type structure(Na3Zr2Si2PO12)*, and I need some 
help regarding a *cation substitution and charge compensation* strategy in 
CP2K.

In my case, I intend to *replace Zr⁴⁺ ions with Mg²⁺ ions* in the 
structure. As you can see, this substitution introduces a *+2 charge 
deficit* per Mg²⁺ ion, and I would like to know the best way to compensate 
for this charge imbalance — possibly by adding *Na⁺ ions*.

My main questions are:

   1. 
   
   *How can I properly introduce additional Na⁺ ions in CP2K to compensate 
   for the charge imbalance caused by Mg²⁺ replacing Zr⁴⁺?*
   Is there a recommended way to control total charge or to specify 
   interstitial Na sites?
   2. 
   
   *How can I determine the most favorable or stable positions for the 
   substituted Mg²⁺ ions and the compensating Na⁺ ions in the NASICON 
   structure?*
   Should I use energy-based relaxation, MD simulations, or site-specific 
   energy comparisons? Is there a common practice for this type of site 
   analysis in CP2K?
   
I would appreciate any guidance, examples, or suggestions regarding how to 
set this up correctly — especially in terms of modeling charge neutrality 
and realistic ion placement.


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