The Hilbert space of the electron-hole pair by considering just one evacuation channel for each charge carrier, with one state at each point (x, y) of the lattice. The coordinates x and y of a given state represent the position of electron and hole in their respective leads. ε(x,y) is the onsite energy of each site of the square lattice and the hopping integrals (C and J) are along the bonds of the square lattice.