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Spectroscopic and optical properties of nanosystems and point defects arediscussed within the framework of Green's function methods. We use an approachbased on evaluating the self-energy in the so-called GW approximation andsolving the Bethe-Salpeter equation in the space of single-particletransitions. Plasmon-pole models or numerical energy integration, which havebeen used in most of the previous GW calculations, are not used. Fouriertransforms of the dielectric function are also avoided. This approach isapplied to benzene, naphthalene, passivated silicon clusters (containing morethan one hundred atoms), and the F center in LiCl. In the latter, excitoniceffects and the $1s \to 2p$ defect line are identified in the energy-resolveddielectric function. We also compare optical spectra obtained by solving theBethe-Salpeter equation and by using time-dependent density functional theoryin the local, adiabatic approximation. From this comparison, we conclude thatboth methods give similar predictions for optical excitations in benzene andnaphthalene, but they differ in the spectra of small silicon clusters. Ascluster size increases, both methods predict very low cross section forphotoabsorption in the optical and near ultra-violet ranges. For the largerclusters, the computed cross section shows a slow increase as function ofphoton frequency. Ionization potentials and electron affinities of moleculesand clusters are also calculated.Comment: 9 figures, 5 tables, to appear in Phys. Rev. B, 200

Optical excitations in organic molecules, clusters, and defects studied by first-principles Green’s function methods