DFTB-Assisted Global Structure Optimization of 13- and 55-Atom Late Transition Metal Clusters

Finding
globally optimal structures of nanoclusters is critically
important to understand their physicochemical properties but remains
prohibitively expensive even with comparatively efficient density
functionals. Semiempirical methods such as density functional tight-binding
(DFTB), on the other hand, offer a better accuracy-efficiency trade-off
but require suitable parametrization. In the present work, we present
a largely automatic scheme where, starting from initial guesses based
on bulk properties, the atomic confinement, and repulsive potentials
are further refined so as to accurately represent the potential energy
landscapes of 13- and 55-atom nanoclusters of the late transition
metals (Ni, Cu, Pd, Ag, Pt, and Au). With the exception of Ni13, Ni55, Cu55, and Ag55,
low-symmetry (often disordered) structures are found to be preferred
over the symmetric icosahedral arrangement. Similar to what has been
previously reported for Au55, the lowest-energy Pt55 structures also appear to contain small cavities below the
outer shell.