(777c) Describing the Diverse Geometries of Gold from Nanoclusters to Bulk − a First-Principles Based Hybrid Bond Order Potential

Gold nano structures are of intense interest for catalysis, sensing, plasmonic, and other applications, and gold nanoclusters (<200 atoms), in particular, exhibit unusual chemistry in the nano-scale. In gold nanoclusters, a fundamental understanding of the atomic structure, size-dependent dimensionality, surface chemistry, and atomic-scale dynamics over timescales spanning tens of nanoseconds holds the key to unraveling structure-property relationships. Gold nanoclusters undergo a fascinating size dependent transition in cluster dimensionality and display a wide range of structural motifs (e.g., planes, hollow cages, tubes, space-filled compact forms). The origin of such transitions has been largely unknown; previous density functional theory (DFT) and spectroscopy studies have speculated that while bond directionality effects predominantly govern structure/chemistry in small Au clusters, long-range dispersive forces probably become important in large clusters, surfaces and bulk condensed phases. A major roadblock has been our inability to perform long time molecular dynamics simulations, and global optimization of structures due to lack of accurate empirical force fields.

In this talk, we introduce a unified ab initio-based hybrid bond order potential (HyBOP), which captures the competition between short-range interactions (bond directionality) and long-range dispersions to accurately predict the size dependent diverse geometries of gold clusters, surfaces and bulk. Our work involves an important methodological development: we employed genetic algorithms to train our HyBOP potential against a large DFT dataset comprising (a) energies of a thousand 13-atom Au clusters, (b) equation of state for various bulk polymorphs of gold, and (c) low-index surfaces of bulk face-centered gold. Our newly developed HyBOP accurately predicts (a) evolution of various structural motifs in Au clusters with increasing size, (b) critical size for transition from planar to globular structures (i.e., dimensionality changes), (c) global energy minimum configurations at various cluster sizes, and (d) thermodynamics, structure, elastic properties, and energetic ordering of bulk condensed phases as well as surfaces, in excellent agreement with DFT calculations and spectroscopic experiments. As a representative test, using long time HyBOP based MD simulations, we identified the atomic scale mechanisms governing the coalescence of icosahedral and planar clusters into pyramids.