Phase transition

[Steven]

Sir
       I want to study pressure induced phase transition of a defect chalcopyrite (low pressure phase). The high pressure phase is a disordered structure with fractional occupancy of atoms ( for this i want to build using supercell 2X2X2). More importantly, I want to calculate the transition pressure  between the two phases.
1.  In order to do this, do i have to build a supercell of the same size for the low pressure phase to compare the enthalpies with the high pressure phase and predict the required transition pressure ?
2. For this, do i have to use LDA or GGA?

[Ulrik G. Vej-Hansen]

1. Yes, I think that would be one way to do it. More specifically, i think I would calculate both structures at a range of pressures, and see when the high-pressure phase becomes lower in energy than the low-pressure phase. If you have a literature reference, I can have a look at it, and help you to sue the same method with ATK and VNL.

2. I would probably use PBE or PBEsol for this - I think LDA could give problems for the high-pressure phase.

[Frank123]

thank you sir
  I have built a structure that has 14 atoms in the conventional cell (low pressure phase).  And for the high pressure phase i am repeating the primitive cell 2x2x2  and then after some substitution and deletion ( for the vacancy), i got exactly 14 atoms which is the same as that of the low pressure conventional cell . Can i compare their enthalpy in this case?

[Frank123]

I mean, in order to compare the enthalpies, does number of atoms need to be same in both the phases?

[Ulrik G. Vej-Hansen]

If they are the same elements, then yes, I would say you can compare the total energies, but the enthalpy also contains the pV term, and I am not sure how you plan to account for that. I am not very familiar with high-pressure phases, so I strongly suggest that you search the literature for papers describing the suggested methodology for doing this comparison thermodynamically correct.