For bands with linear dispersion E ∝ ∝ k such as with photons or electrons in graphene, the group velocity is fixed, i.e. electrons travelling with parallel with k to the force direction F cannot be accelerated and the diagonal elements of M inert are obviously zero. However, electrons travelling with a component perpendicular to the force can be accelerated in the direction of the force, and the off-diagonal elements of M inert are non-zero. In fact the off-diagonal elements scale inversely with k, i.e. they diverge (become infinite) for small k. This is why the electrons in graphene are sometimes said to have infinite mass (due to the zeros on the diagonal of M inert) and sometimes said to be massless (due to the divergence on the off-diagonals).