Global optimization of neutral and charged 20- and 55-atom silver and gold clusters at the DFTB level

Abstract

The global optimization of metallic clusters is an important topic because nanoclusters exhibit structure-dependent properties. In this paper, we present a global optimization study of Ag20, Au20, Ag55 and Au55 in their neutral and charge states (−1, 0, +1) conducted using a Parallel-Tempering Molecular Dynamics algorithm at the DFTB level without pre-screening. For Au20, Ag20 and their ions, the present DFTB low energy structures are in good agreement with previously published calculations and experimental data. In the case of Ag55- and Au55-, the present study is consistent with photo-electron detachment experiments suggesting highly symmetric icosahedral structures for silver and more disordered morphologies for gold. The present results are also compatible with trapped ion electron diffraction experiments and calculations for Ag55+ and Ag55-. We report low-energy isomers of Au55 exhibiting cavities below their external shell. This work quantitatively confirms the relevance of DFTB for structure calculation of noble metal clusters. Furthermore, it also demonstrates the feasibility of global optimization using DFTB, without pre-screening through classical potential, for sizes up to a few tens of atoms and for different charge states.