Collective electronic excitations in ${\mathrm{C}}_{60}$ crystals

We present a theoretical study of the electronic excitations in fullerene crystals by calculating the density-density correlation function in a fully nonlocal linear response theory. Our results indicate that the collective features associates with the π→${\mathrm{\pi }}^{\mathrm{*}}$ transitions show strong anisotropic properties, with peaks changing by as much as 0.7 eV in different directions. Meanwhile, the calculated mode dispersion exhibits rather weak wave-number dependence along a given direction, in general agreement with experimental results. The oscillator strength also shows anisotropic behavior, with significant weight redistribution for different directions. We also analyze this system in terms of a classical point-dipole array model, and show that this simple model approximates well the quantum results.