Local-basis quasiparticle calculations and the dielectric response function of Si clusters

Abstract

We present an ab initio computational scheme for evaluating the dielectric response function of Si clusters. All calculations are carried out employing a basis of localized atomiclike orbitals and including quasiparticle corrections. The self-energy operator is evaluated in the GW approximation, with a full frequency dependence for the dielectric matrix. The approach is convenient and computationally optimal for the calculation of optical properties of complex systems lacking full periodicity, such as surfaces and clusters. We present here the quasiparticle-level structure for ${\mathrm{Si}}_{20}$ and ${\mathrm{Si}}_{60}$ clusters and discuss the sensitivity of their optical properties to quasiparticle corrections. We find that the optical gap is larger than in bulk silicon, clearly the net result of size quantization over structural disorder.