Let us emphasize that the apparently simple case of a genuine, periodic crystal can be challenging also when the crystal is a metal. Recall that, for periodic crystals, using a supercell is mathematically equivalent to sampling the Brillouin zone with the regular k-point grid 713 ; the advantage of the latter approach is that it is far more efficient from a computational viewpoint. In most calculations, a relatively coarse k-point grid is used to further reduce the computational burden (say, 3 × 3 × 3 for insulators and 7 × 7 × 7 for metals). This approach is usually sufficient for insulators because the integrands are periodic, analytic, and weakly oscillating over the Brillouin zone for all relevant physical observables, but far from sufficient 714 for metals with complicated Fermi surfaces. Smearing techniques 715 at a fictitious pos-itive temperature (possibly higher than the melting temperature of the metal) help to some extent, 716 but do not fully solve the problem. It appears that many computational results on metals reported in the literature cannot be considered as converged with respect to k-point discretization.