use of PBC allows one to reduce the system size dramatically. For bulk materials this even allows simple systems, such as face-centered cubic Cu or Ni, to be represented using single atom unit cells. A useful side effect of such reduced cells is that it is easily possible and relatively cheap to store the CDD $\rho_{system}(\mathbf{r})$ on a three dimensional grid covering the unit cell, and thus fully describing the entire infinite system. The use of such precalculated charge density grids speeds up the Hirshfeld method significantly, since there is no more need to calculate the charge density at any given grid point starting from the wave function of the system. This also makes the implementation independent of the code used to generate the system. The drawback, however, is a slightly reduced accuracy. Since the charge density grid has a finite resolution, interpolation between the stored grid points is needed. This effect is discussed in the following section.\
In chemistry, it is usually sufficient to consider only the valence electrons to describe the interactions between atoms. The core electrons are often considered inert as a result, and are kept frozen during calculations, greatly reducing the computational cost for heavy atoms. 