As the question states, I'd like to study phonon behavior at the interface of two semiconducting materials (GaN and diamond), using DFT (or DFPT) within the framework of VASP (the program I use). I am familiar with the method of calculating force constants and then running them through a program like phonopy to get the DOS and phonon band structure, but I'm not sure if that is sufficient for an interface. Often times the supercell is symmetrically enlarged in the phonopy tutorials, so I wasn't sure what might change when I have a supercell that is already 1x1x6 (elongated supercell slab with 3 unit cells of each material, both surface passivated with pseudo-hydrogen).

So, what is the best methodology to go about doing this to better understand thermal transport properties of the interface?? I plan to also include some analytical models like the Boltzmann transport equation, so any resources or suggestions are welcome and greatly appreciated.