Dear All, this is my first post on this forum and hopefully I can get what I want. . I am trying to build the band structure of graphene using the tight-binding method based on slater-koster correction. I use a special code to construct both Hamiltonian and overlapping matrix. I have seen when I try to diagonalize the final matrix to find the eigenvalues, the eight eigenvalues are same even if I move from Gamma- K - M. I presume there is a reason for that I don't understand it. So please I need your help to solve my issue.