+Due to the Bloch theorem only a finite number of electronic wave functions need to be calculated for a periodic system.
+However, to calculate quantities like the total energy or charge density, these have to be evaluated in a sum over an infinite number of $\vec{k}$ points.
+Since the values of the wave function within a small interval around $\vec{k}$ are almost identical, it is possible to approximate the infinite sum by a sum over an affordable number of $k$ points, each representing the respective region of the wave function in $\vec{k}$ space.
+Methods have been derived for obtaining very accurate approximations by an intergration over special sets of $\vec{k}$ points \cite{}.
+If present, symmetries in reciprocal space may further reduce the number of calculations.
+For supercells, i.e. repeating unit cells that contain several primitive cells, sampling of the Brillouin zone restricted to the $\Gamma$ point can be quite accurately used, which is equivalent to calculating a single primitive cell using multiple $\vec{k}$ points.
+