  In this work we argue that it is important to make a proper choice of the
unit cell within tight-binding theory if one aims to compute the optical matrix
elements. By considering zigzag graphene nanoribbons, we demonstrate that the
band structure is insensitive to the choice of the unit cell. However, if the
chosen unit cell does not incorporate the symmetries of the system, wrong
values of optical matrix elements are obtained, leading to the violation of
well-known selection rules. This result holds irrespective of the formalism
utilized for calculating the optical matrix elements.
