If you are interested in diagonalizing Heisenberg or Hubbard Hamiltonian
imposing PERIODIC BOUNDARY CONDITION, and you are trying to obtain
low-lying eigenstates of the models IN EACH MOMENTUM SPACE, please
read the following articles. (From my thesis.... what a premature article !!)

p00, p01, p02, p03, p04, p05, p06, p07, p08, p09,

p10, p11, p12, p13, p14, p15, p16, p17, p18, p19,

p20, p21, p22, p23, p24, p25, p26, p27