Molecular dynamics simulations of cluster structure

We have an ongoing collaboration with Dr Shaun Hendy at Industrial Research Limited on simulations of cluster structure. 

The structure of an atomic cluster often differs from that of the corresponding bulk material. In such a cluster, the number of surface atoms is comparable to the number of interior atoms, and consequently, the surface energy plays an important role in determining the overall structure. For example, regular noncrystalline structures with fivefold axes of symmetry, such as icosahedra and decahedra, are known to occur in gold and a variety of other face-centered cubic (fcc) metals. Such structures are comprised of deformed fcc tetrahedral units where adjacent tetrahedral faces meet at a twin plane. The energy cost of twinning at the inner tetrahedral faces, and the strain energy in the deformed tetrahedra, is overcome by the resulting energetically favorable close-packed outer faces.

lead-atom cluster

The lead-atom cluster shown above is icosahedral with an unusual surface-reconstruction that stabilises the structure.

The structure of a cluster, while not only of fundamental interest, is also a key determinant of many of its properties. However the delicate balance between surface and internal energies often produces a complex dependence of structure upon cluster size. Eventually, as the size of a cluster increases, the bulk structure must win out, but at sizes below this, clusters can assume a variety of regular non-crystalline structures. 

One way to study this complex behaviour is to use molecular dynamics to simulate clusters on a computer. The simulations become slower and slower as the number of atoms in a cluster increases, so high-performance computing is an important tool for studying all but the smallest clusters. We use a 46-processor Athlon MP 2000+ linux cluster for our simulations.

Recent publications on MD simulations of cluster structure: