How to calculate the bandstructure of Twisted Bilayer Graphene

bauketek

Dear PF members,

I have difficulty in finding the bandstructure of twisted bilayer graphene by using tight binding method.
I can't understand in interlayer hamiltonian part described in this paper.

Phys. Rev. B 82, 121407(R) (2010)
"Flat bands in slightly twisted bilayer graphene: Tight-binding calculations"

Could you please explain it to me?

Best regards