Dear all, I create Hamiltonian H for a graphene lattice with nn-hopping only, no leads, i.e. closed system. Each a (b) sublattice site is located at r=i*a_1+j*a_2 (r=i*a_1+j*a_2+d), with i,j being integers, a_1,2 being Bravais lattice vectors, and d a vector pointing from a to b sublattice. Hence each site is marked with |i,j,a or b>. My question is, once the nn-hoppings are set, how do I locate the position in H belonging to specific matrix element between two sites, say <2,5,a|H|1,2,b>? My goal is to plot a dispersion relation E(k_y) of such a closed system with periodic boundary conditions in y-direction (I did it by hand) and hard-wall in x-direction. I guess I need to do the Fourier transform in y-direction by hand. Or, are there some built-in functions to do this? Thank you for eventual answer or pointing me to duplicate question. Tibor