An inelastic scattering mechanism, like phonon scattering, makes an electron emit/absorb energy of ℏωq. The number of unique energy values representable by the discrete k-values is determined by the number of unique sum of squares:
The energy gained/lost through phonon absorption/emission (ℏωg) in general will not exactly correspond to an interval in the discretized energy grid and therefore will be rounded up/down. The approximation introduced by this rounding is below 1meV for most points in the energy grid. However, at higher energy values, where the energy grid is coarser, the energy gained/lost by optical phonon can be under- or overestimated.