Ab Initio Calculations of Clusters

Abstract

Both ab initio quantum chemistry and the investigation of atomic or molecular clusters are fields of active research nowadays. This contribution presents on one hand some advances in making programs efficient for calculations of large systems, on the other it shows some fruitful combinations of calculated and experimental results. Respecting the context of the CFN summer school, it is addressed rather to non-quantum chemists, to give an insight into contemporary quantum chemical possibilities in calculations of clusters as well as to show the strengths and weaknesses of several methods. In the following chapter the very basics of quantum chemistry are discussed (as far as necessary for what follows), next some details of implementation to increase efficiency of HF, DFT and MP2 algorithms are shown. In the last section three different applications of quantum chemical methods for atomic and for molecular clusters are presented. Firstly, as an example for the HF+MP2-method, the determination of the structure and bonding situation in negatively charged clusters of water molecules is discussed, secondly a similar study of anionic gold clusters, but this time by means of density functional theory (DFT) is presented. In both cases, by combining theory and experiment results are obtained which would not have been found by calculations alone or only by experiments. The last application presented here is a DFT study on clusters of magnesium (up to Mg₃₀₉), focussing on the validity of the simple models of electronic and atomic shells leading to ’magic’ atom and electron numbers.