Optical properties of superconducting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and insulating ${\mathrm{K}}_6$${\mathrm{C}}_{60}$

The electronic structures and the linear optical properties of superconducting fcc ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and insulating bcc ${\mathrm{K}}_6$ ${\mathrm{C}}_{60}$ are calculated by means of a first-principles method within the local-density approximation. The calculated optical properties for these two crystals as well as those of fcc ${\mathrm{C}}_{60}$ are in very good agreement with recent electron-energy-loss spectra measurements.