Help with Charge Compensation and Site Selection in NASICON Structure (Zr → Mg Substitution)

[Sam Hesamie]

Dear all
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.