FCC-to-BCC phase transition in NaCl, Buckingham or Lennard-Jones potentials? Background
The transformation from B1 (face centered cubic (FCC) type) to B2 (body centered cubic (body centered cubic (BCC) type) structures is one of the best documented high pressure phase transition occurring in alkali halides (e.g. see here). 
I am studying phase transitions as part of a PhD in High Performance Computing applications in Biology/Physics/Engineering. Currently, I would like to address several questions by implementing molecular dynamics simulations. 
Consider that we have an FCC NaCl crystal of about 200 atoms, with Buckingham potential parameters: 


Questions


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*If we vary conditions (temperature/pressure), such that an FCC-BCC phase transition occurred, how would we know the BCC lattice formed? 

*How would we know Buckingham potentials effectively model the interionic interactions? 

*What would we expect if Lennard-Jones potentials were used to model the interactions instead?

*What conditions are required for the phase transition (FCC-to-BCC)? (e.g. temperature/pressure)

 A: 
  
*
  
*If we vary conditions (temperature/pressure), such that an FCC-BCC phase transition occurred, how would we know the BCC lattice formed?
  

In this paper Zhang & Chen used Möbius pair-potentials to model NaCl phase transitions in an MD simulation, so I am not sure how relevant it is to your question, but they use several indicators of the FCC-BCC transition. Specifically, in response to high-external pressure, they observe abrupt changes Changes in:


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*Cell lengths and Cell angles 

*Coulomb energy (Madelung constant)

*First-nearest-neighbour distance of atoms (search 'Radial distribution function')






  
*What would we expect if Lennard-Jones potentials were used to model the interactions instead?
  

Buckingham potentials are a simplified version of Lennard-Jones potentials. In terms of physics, all I can say, is that there may be "un-physically" high attractions if two ions are too close to one another. 
However, in terms of your PhD project, it may be better to use Buckingham potentials due to cheaper computation.



  
*What conditions are required for the phase transition (FCC-to-BCC)? (e.g. temperature/pressure)
  

In the same paper, conditions for phase transition of FCC-to-BCC were: 


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*30 GPa (pressure)

*600 K (temperature)

*256 Na+ ions, 256 Cl- ions

