I have known:
(1) A Hamiltonian, say, H(k), where k is the crystal momentum.
(2) An appropriate complete basis set {a_1,a_2,a_3…}.
(3) Some symmetric operators {A,B,…} which commute with H(k), i.e. [A,H]=[B,H]=...=0.
Of course, by calculation, I can get any matrix element of H(k), i.e...