Electron excitations in solidC60: Energy gap, band dispersions, and effects of orientational disorder

Electron excitation energies and photoemission spectra in undoped, solid ${\mathrm{C}}_{60}$ are calculated using a quasiparticle approach. The effects of orientational disorder and electron correlations are studied. We find a band gap of 2.15 eV, in good agreement with experiment, and ∼1 eV widths for the highest-occupied-molecular-orbital (HOMO) and lowest-unoccupied-molecular-orbital (LUMO) bands. Calculated angle-resolved, inverse photoemission spectra for the LUMO bands show very little angular dependence, explaining recent experimental work on epitaxial thin films. The present results suggest that undoped, solid ${\mathrm{C}}_{60}$ is a standard band insulator.