The main body of this thesis is devoted to the investigation of the quasiparticle spectra of neutral gold clusters by using the GW method. We focus on the study of small clusters AuN (N=1-8) (including the Au atom), as well as two larger quasi two-dimensional clusters, the tetrahedral Au20 cluster and the cagelike Au32 cluster. The Quantum Espresso density-functional package is employed to perform electronic structure calculations. In particular, the GWL module of Quantum Espresso is used for the GW parts of the calculations. Our implementation of the GW method has incorporated recently introduced algorithms which address the main computational difficulties involved in traditional GW calculations and can significantly enhance the scope of GW calculations.
Two specific aspects are considered concerning the GW calculations of gold clusters: the Au semicore 5s and 5p states and the open-shell character of some symmetric molecular structures. We find that an explicit inclusion of the Au semicore 5s and 5p states in the valence manifold is essential to achieve a satisfactory accuracy in the calculated quasiparticle spectra. Based on this observation, we use a recently developed simplified approach to account for the effects of the Au semicore 5s and 5p states without including them fully in the GW calculations. By doing so, a significant speed up of the calculations is obtained, while the accuracy of the computed spectra is not compromised. This simplified approach makes the GW method fit to the study of larger gold clusters, such as Au20 and Au32, which can not be tackled easily using more conventional approaches. The electronic structure of open-shell molecules is hardly addressed in traditional GW calculations. However, in the study of small clusters, one can frequently meet symmetric structures with open-shells. We show how the electronic excitation properties of open-shell molecules can be studied by making some compromise in the GW calculations of these molecules.
