Modeling calcium and strontium clusters with many-body potentials

Many-body atomistic potentials, of the Murrell-Mottram (MM) type, obtained by fitting properties of solid phases of calcium and strontium [J. E. Hearn, R. L. Johnston, S. Leoni, and J. N. Murrell, J. Chem. Soc. Faraday Trans. 92, 425 (1996)], have been used to study the structures, stabilities, and growth modes of Ca and Sr clusters. Full structure optimization on small clusters (2-20 atoms) leads to structures involving the fusion of tetrahedral units, and predicts icosahedral cluster growth. Radial relaxation studies on icosahedral, truncated decahedral, cuboctahedral, and rhombic dodecahedral geometric shell clusters, lead to the prediction that icosahedral structures are preferred until around 32 000 (Ca) and 128 000 (Sr), whereupon the fcc-like cuboctahedral clusters become preferred These results are consistent with experimental findings. A detailed analysis has been performed of the binding energies and radial expansion factors of each set of symmetry equivalent atoms (subshell). As for Lennard-Jones clusters, multishell icosahedral Ca and Sr clusters are predicted to undergo significant core compression, resulting in low binding energies for the central atom and inner shells.